Detection of large-scale X-ray bubbles in the Milky Way halo.

The halo of the Milky Way provides a laboratory to study the properties of the shocked hot gas that is predicted by models of galaxy formation. There is observational evidence of energy injection into the halo from past activity in the nucleus of the Milky Way1-4; however, the origin of this energy (star formation or supermassive-black-hole activity) is uncertain, and the causal connection between nuclear structures and large-scale features has not been established unequivocally. Here we report soft-X-ray-emitting bubbles that extend approximately 14 kiloparsecs above and below the Galactic centre and include a structure in the southern sky analogous to the North Polar Spur. The sharp boundaries of these bubbles trace collisionless and non-radiative shocks, and corroborate the idea that the bubbles are not a remnant of a local supernova5 but part of a vast Galaxy-scale structure closely related to features seen in γ-rays6. Large energy injections from the Galactic centre7 are the most likely cause of both the γ-ray and X-ray bubbles. The latter have an estimated energy of around 1056 erg, which is sufficient to perturb the structure, energy content and chemical enrichment of the circumgalactic medium of the Milky Way.

A common mechanism for the mechanosensitive regulation of apoptosis in different cell types and for different mechanical stimuli.

In all kinds of tissue cells are influenced by mechanical forces. In vivo fibroblasts are exposed to mechanical tension and endothelial cells are subjected directly to hemodynamic flow. It has been shown that disturbance of the mechanical stimulus leads to apoptosis by induction of an autocrine loop with thrombospondin-1 as ligand and an integrin/integrin associated protein (CD47) complex as receptor. In the present study the nature of the mechanical stimulus has been exchanged for these two cell types. If fibroblasts are subjected to laminar flow apoptosis decreases about 20-fold whereas turbulence leads to an significant increase compared with the static conditions. If endothelial cells grown on thin silicone membranes are exposed to permanent and pulsatile uniaxial strain, the cells are completely devoid of apoptosis. The thrombospondin-1 secretion as well as the expression of CD47 occurs exclusively under mechanical relaxation respectively turbulence. So different types of cells seem to share a common sense deciding whether a mechanical stimulus induces or suppresses apoptosis and use a common molecular machinery for the regulation of the process.

Shear stress induces apoptosis in vascular smooth muscle cells via an autocrine Fas/FasL pathway.

Endothelial lesions may lead to the exposure of vascular smooth muscle cells (VSMCs) to the blood flow. In such circumstances VSMCs are exposed to shear stress, an extraordinary mechanical stimulus for this type of cells. Rat VSMCs are cultivated in normal tissue culture plates (statically) or in a cone-plate viscometer (dynamically). Dynamic cultivation leads to a great increase of apoptosis. Immunofluorescence reveals the shear-stress-dependent expression of fas. Apoptosis can be induced by addition of fas ligand-a process which can be blocked by antibodies against either fas or fas ligand. Conditioned medium of dynamically cultivated VSMCs contains fas ligand as the only active apoptosis inducing activity. Apoptosis can be blocked by caspase inhibitors. So the exposure of VSMCs to shear stress leads to apoptosis by the establishment of an autocrine loop of fas and fas ligand-a potential mechanism for the prevention of narrowing of vessel diameter by VSMC proliferation.

Mechanosensitive induction of apoptosis in fibroblasts is regulated by thrombospondin-1 and integrin associated protein (CD47).

Fibroblasts are cultured in three-dimensional collagen matrices to investigate the effect of mechanical tension on the regulation of apoptosis. Under the influence of mechanical loading, the cells show little apoptosis whereas releasing of tension leads to an increase up to tenfold during the first 24 h and remains constant for further 48 h. An autocrine loop of the integrin alpha(V)beta(3)/CD47 receptor complex and thrombospondin-1 is identified as the molecular coupling device between mechanical loading and apoptosis: The integrin alpha(V)beta(3) is expressed under mechanical loading as well as unloading whereas the CD47 could only be identified after the release of tension. The secreted thrombospondin binds to the active receptor and induces apoptosis. The presented mechanosensitive regulation of apoptosis in fibroblast cultures could be an essential mechanism for the regression of the granulation tissue by apoptosis in the process of wound healing.

The role of thrombospondin-1 in apoptosis.

The thrombospondins are a family of extracellular proteins that participate in cell-to-cell and cell-to-matrix communication. They regulate cellular phenotype during tissue genesis and repair. Five family members, each representing a separate gene product, probably exist in most vertebrate species. Like most extracellular proteins, the thrombospondins are composed of several structural domains that are responsible for the numerous biological functions that have been described for this protein family. Considerable progress has been made towards understanding the function of thrombospondins. The role of thrombospondin in the process of apoptosis or programmed cell death has recently come into focus. In this review we will concentrate on the role of thrombospondin-1 in the broad field of apoptotis research.

Proatherogenic flow conditions initiate endothelial apoptosis via thrombospondin-1 and the integrin-associated protein.

Recently it has been shown that vascular endothelial cells (EC) are completely devoid of apoptosis if cultivated under a steady laminar flow and that apoptosis is induced by turning off the flow. An autocrine loop of thrombospondin-1 (TSP-1) and the alpha(v)beta(3) integrin/integrin-associated protein (IAP) complex has been identified as the molecular coupling device between flow and apoptosis. Lack of blood flow is a rare and mostly transient phenomenon whereas irregular flow conditions are permanently present at arterial bifurcations and sites of abnormal vessel morphology. Irregular flow conditions are established here either by the action of a cone-and-plate type flow apparatus generating a uniform turbulent flow or in a flow chamber by insertion of a local hindrance creating a zone of unsteady laminar flow with vortex formation and lowered shear stress. In both cases apoptosis is induced either throughout the entire monolayer or restricted to the locally defined area. Flow disturbance and apoptosis are coupled by the described autocrine loop of TSP-1 and the integrin/IAP receptor complex. In vivo atherosclerotic lesions occur predominantly at sites of flow irregularities, which are thought to be pro-atherogenic. Thus we propose a key role of the identified mechanosensitive apoptosis induction for the initiation of atherosclerosis.

Vascular endothelial cells express a functional fas-receptor due to lack of hemodynamic forces.

The fas system is present in atherosclerotic lesions. However, its role in the initiation and progression is still unclear. Here we show that in endothelial cells (EC) the expression of the fas receptor is regulated by flow conditions. The EC of the vascular system are regularly exposed to a range of hemodynamic forces with great impact on cellular structures and functions. Recently it was reported that in endothelial cells the lack of hemodynamic forces as well as irregular flow conditions trigger apoptosis by induction of a mechanosensitive autocrine loop of thrombospondin-1 and the alpha(V)beta(3) integrin/integrin-associated protein complex. Here we show that EC cultivated under regular laminar flow conditions are devoid of the fas-receptor whereas cultivation under static conditions as well as under turbulence leads to its expression. Stimulation of the fas-receptor by its ligand increases the amount of apoptotic cells by twofold; the increase can be prevented by blocking the fas-receptor. The availability of the expressed fas receptor for stimulation by its ligand hints at a role as a tool for progression of atherosclerosis.

Integrin-associated protein and thrombospondin-1 as endothelial mechanosensitive death mediators.

Recently, it was reported that the offset of hemodynamic forces induces an unusual pattern of apoptosis in vascular endothelium (1). Although the apoptotic trigger covers all cells and is maintained for a longer time period, only few cells become apoptotic. So, in contrast to common apoptosis inducers, the lack of hemodynamic forces initiates only a low basal level of apoptosis, however steadily increases with time, this way preventing the complete vessel destruction upon an only transient offset of blood flow. The molecular means by which the mechanical stimulus and apoptosis are smoothly coupled have now been identified as an autocrine loop of thrombospondin-1 (TSP-1) and the alpha(v)beta(3) integrin/integrin-associated protein (IAP) complex as its receptor. Vascular EC (EC) secrete TSP-1 only in postconfluent static monolayers and not under flow. This also holds true for the IAP whereas the alpha(v)beta(3) integrin is present under static conditions, as well as under flow, assigning the IAP an essential and new switch function in the receptor complex.

Apoptosis induced by lack of hemodynamic forces is a general endothelial feature even occuring in immortalized cell lines.

Confluent monolayers of primary endothelial cells display a high viability and an apparently constant cell density. However upon prolonged cultivation the monolayer degenerates with increasing numbers of senescent cells finally representing the whole culture. Recently we showed that lack of hemodynamic forces induces apoptosis in organ cultures as well as in confluent monolayers of human umbilical cord vein endothelial cells (HUVEC). The apoptosis started at a low level and was counteracted by a continuous proliferation of the remaining cells. Here we show that the induction of apoptosis by lack of hemodynamic forces is a general characteristic of vascular endothelial cells, valid for endothelial cells from various organs and species: human umbilical cord vein endothelial cells (HUVEC), human microvascular placental endothelial cells (HPEC) and bovine aorta endothelial cells (BAEC). Furthermore apoptosis due to the lack of hemodynamic forces can also be induced in various endothelial cell lines: EA.hy 926 derived from HUVEC and PBMEC-A1 derived from PBMEC. However degeneration of confluent monolayers does not occur with these cell lines even in monolayers kept for several weeks. This indicates that the degeneration of normal endothelial cell monolayers is caused by depletion of the proliferation potential of the endothelial cells.

The use of a conventional tissue culture plate as an optically accessible perfusion chamber for in situ assays and for long-term cultivation of mammalian cells.

An alternative culture system has been developed based on a conventional tissue culture plate (3.5 cm diameter) which is changed into a closed perfusion chamber. The system can easily be scaled up from one to several chambers. The shape and the size of the area of cell growth may be designed to individual experimental demands. The whole culture chamber is optically accessible, so cell growth and morphology can be evaluated by light microscopy. Furthermore the cellular physiology can be characterised by any fluorimetric assay using a bottom type fluorescence reader. A peristaltic pump sustains a constant medium flow through the chamber thus creating true homeostasis. The use of HPLC-valves and connectors allows the switching between different media or assay solutions. Thus it is possible to perform in situ assays also measuring transient effects. A protocol for vitality tests using calcein-AM is worked out for an adherent cell line and for a suspension cell line. The lower detection limits are 7 × 10(2) cells cm(-2) for the adherent cells and 5 × 10(4) cells mL(-1) for the suspension cells. The upper limits are 1-2 × 10(5) cells cm(-2) respectively 8 × 10(6) cells mL(-1).

Lack of hemodynamic forces triggers apoptosis in vascular endothelial cells.

The endothelial cells (EC) of the vascular system are regularly exposed to a range of hemodynamic forces with great impact on cellular structures and functions. A shear-stress-dependent modulation has been reported for various endothelial functions, and first attempts were made to characterize the nature of the shear-stress-sensitive receptor. Apoptosis has been induced in cultures of vascular EC upon application of various stimuli characteristic of traumatic or pathogenic events. However, no link between apoptosis and mechanical stimulation has been reported so far-neither for vascular EC nor for other types of cells. Here we show that the lack of hemodynamic forces triggers apoptosis in endothelial cells, hinting at mechanical forces as essential stimuli for the maintenance of blood vessels.