Activation of Integrins by Urea in Perfused Rat Liver

High concentrations of urea were shown to induce a paradoxical regulatory volume decrease response with K⁺ channel opening and subsequent hepatocyte shrinkage (Hallbrucker, C., vom Dahl, S., Ritter, M., Lang, F., and Häussinger, D. (1994) Pflügers Arch. 428, 552–560), although the hepatocyte plasma membrane is thought to be freely permeable to urea. The underlying mechanisms remained unclear. As shown in the present study, urea (100 mmol/liter) induced within 1 min an activation of β₁ integrins followed by an activation of focal adhesion kinase, c-Src, p38^MAPK, extracellular signal-regulated kinases, and c-Jun N-terminal kinase. Because α₅β₁ integrin is known to act as a volume/osmosensor in hepatocytes, which becomes activated in response to hepatocyte swelling, the findings suggest that urea at high concentrations induces a nonosmotic activating perturbation of this osmosensor, thereby triggering a volume regulatory K⁺ efflux. In line with this, similar to hypo-osmotic hepatocyte swelling, urea induced an inhibition of hepatic proteolysis, which was sensitive to p38^MAPK inhibition. Molecular dynamics simulations of a three-dimensional model of the ectodomain of α₅β₁ integrin in water, urea, or thiourea solutions revealed significant conformational changes of α₅β₁ integrin in urea and thiourea solutions, in contrast to the simulation of α₅β₁ in water. These changes lead to an unbending of the integrin structure around the genu, which may suggest activation, whereas the structures of single domains remained essentially unchanged. It is concluded that urea at high concentrations affects hepatic metabolism through direct activation of the α₅β₁ integrin system.     

The Drosophila LEM-domain protein MAN1 antagonizes BMP signaling at the neuromuscular junction and the wing crossveins

BMP signaling responses are refined by distinct secreted and intracellular antagonists in different cellular and temporal contexts. Here, we show that the nuclear LEM-domain protein MAN1 is a tissue-specific antagonist of BMP signaling in Drosophila. MAN1 contains two potential Mad-binding sites. We generated MAN1^ΔC mutants, harbouring a MAN1 protein that lacks part of the C-terminus including the RNA recognition motif, a putative Mad-binding domain. MAN1^ΔC mutants show wing crossvein (CV) patterning defects but no detectable alterations in nuclear morphology. MAN1^ΔC pupal wings display expanded phospho-Mad (pMad) accumulation and ectopic expression of the BMP-responsive gene crossveinless-2 (cv-2) indicating that MAN1 restricts BMP signaling. Conversely, MAN1 overexpression in wing imaginal discs inhibited crossvein development and BMP signaling responses. MAN1 is expressed at high levels in pupal wing veins and can be activated in intervein regions by ectopic BMP signaling. The specific upregulation of MAN1 in pupal wing veins may thus represent a negative feedback circuit that limits BMP signaling during CV formation. MAN1^ΔC flies also show reduced locomotor activity, and electrophysiology recordings in MAN1^ΔC larvae uncover a new presynaptic role of MAN1 at the neuromuscular junction (NMJ). Genetic interaction experiments suggest that MAN1 is a BMP signaling antagonist both at the NMJ and during CV formation.     

Dopamine Induces Ca2+ Signaling in Astrocytes through Reactive Oxygen Species Generated by Monoamine Oxidase

Dopamine is a neurotransmitter that plays a major role in a variety of brain functions, as well as in disorders such as Parkinson disease and schizophrenia. In cultured astrocytes, we have found that dopamine induces sporadic cytoplasmic calcium ([Ca²⁺]_c) signals. Importantly, we show that the dopamine-induced calcium signaling is receptor-independent in midbrain, cortical, and hippocampal astrocytes. We demonstrate that the calcium signal is initiated by the metabolism of dopamine by monoamine oxidase, which produces reactive oxygen species and induces lipid peroxidation. This stimulates the activation of phospholipase C and subsequent release of calcium from the endoplasmic reticulum via the inositol 1,4,5-trisphosphate receptor mechanism. These findings have major implications on the function of astrocytes that are exposed to dopamine and may contribute to understanding the physiological role of dopamine.     

Yersinia YopP-induced apoptotic cell death in murine dendritic cells is partially independent from action of caspases and exhibits necrosis-like features

Yersinia outer protein P (YopP) is a virulence factor of Yersinia enterocolitica that is injected into the cytosol of host cells where it targets MAP kinase kinases (MKKs) and inhibitor of κB kinase (IKK)-β resulting in inhibition of cytokine production as well as induction of apoptosis in murine macrophages and dendritic cells (DC). Here we show that DC death was only partially prevented by the broad spectrum caspase inhibitor zVAD-fmk, indicating simultaneous caspase-dependent and caspase-independent mechanisms of cell death induction by YopP. Microscopic analyses and measurement of cell size demonstrated necrosis-like morphology of caspase-independent cell death. Application of zVAD-fmk prevented cleavage of procaspases and Bid, decrease of the inner transmembrane mitochondrial potential ΔΨ_m and mitochondrial release of cytochrome c. From these data we conclude that YopP-induced activation of the mitochondrial death pathway is mediated upstream via caspases. In conclusion, our results suggest that YopP simultaneously induces caspase-dependent apoptotic and caspase-independent necrosis-like death in DC. However, it has to be resolved if necrosis-like DC death occurs independently from apoptotic events or as an apoptotic epiphenomenon.     

Phosphorylation of the cyclin-dependent kinase inhibitor p21Cip1 on serine 130 is essential for viral cyclin-mediated bypass of a p21Cip1-imposed G1 arrest

K cyclin encoded by Kaposi's sarcoma-associated herpesvirus confers resistance to the cyclin-dependent kinase (cdk) inhibitors p16Ink4A, p21Cip1, and p27Kip1 on the associated cdk6. We have previously shown that K cyclin expression enforces S-phase entry on cells overexpressing p27Kip1 by promoting phosphorylation of p27Kip1 on threonine 187, triggering p27Kip1 down-regulation. Since p21Cip1 acts in a manner similar to that of p27Kip1, we have investigated the subversion of a p21Cip1-induced G1 arrest by K cyclin. Here, we show that p21Cip1 is associated with K cyclin both in overexpression models and in primary effusion lymphoma cells and is a substrate of the K cyclin/cdk6 complex, resulting in phosphorylation of p21Cip1 on serine 130. This phosphoform of p21Cip1 appeared unable to associate with cdk2 in vivo. We further demonstrate that phosphorylation on serine 130 is essential for K cyclin-mediated release of a p21Cip1-imposed G1 arrest. Moreover, we show that under physiological conditions of cell cycle arrest due to elevated levels of p21Cip1 resulting from oxidative stress, K cyclin expression enabled S-phase entry and was associated with p21Cip1 phosphorylation and partial restoration of cdk2 kinase activity. Thus, expression of the viral cyclin enables cells to subvert the cell cycle inhibitory function of p21Cip1 by promoting cdk6-dependent phosphorylation of this antiproliferative protein.     

Pericytes and vascular stability

Newly formed endothelial tubes are initially unstable and subsequently become stabilized through the formation of a perivascular matrix and the association with pericytes. The presence of pericyte per se is not sufficient for vascular stability. Instead, specific qualities of the cells are required that seem to correlate with marker expression and the nature of the endothelial-pericyte contacts. Most likely, specific intercellular signals are required as mediators of endothelial and pericyte cell function and vascular stability. Several ligand-receptor systems have been implicated in endothelial-pericyte interactions. Here, we discuss the role of some of these signaling systems in the regulation of vascular stability.     

Profiling of structurally labile oxylipins in plants by in situ derivatization with pentafluorobenzyl hydroxylamine

A GC-MS-based method for the simultaneous quantification of common oxylipins along with labile and highly reactive compounds based on in situ derivatization with pentafluorobenzyl hydroxylamine to the corresponding O-2,3,4,5,6-pentafluorobenzyl oximes (PFB oximes) is presented. The approach covers oxo derivatives such as jasmonic acid (JA), 12-oxophytodienoic acid (OPDA), certain phytoprostanes, unsaturated oxo-acids, oxo-hydroxy acids, and aldehyde fragments from the polar head of fatty acids. In the positive electron impact-MS mode, the PFB oximes display characteristic fragment ions that greatly facilitate the identification of oxylipins in complex matrices. In addition, the fluorinated derivatives allow a highly selective and low-background analysis by negative chemical ionization. Besides showing the general value of the method for the identification of a broad range of oxylipins (18 examples), we also demonstrate sensitivity, linearity, and reproducibility for the quantification of JA, OPDA, 11-oxo-9-undecenoic acid, and 13-oxo-9,11-tridecadienoic acid. The efficiency of the method is demonstrated by differential profiling of these four oxylipins in lima bean leaves after mechanical wounding and feeding by the herbivore Spodoptera littoralis. Caterpillar feeding induced several oxylipins, whereas after wounding only the level of JA increased. The rapid in situ derivatization prevents the isomerization of cis-JA to trans-JA. The resting level of JA in lima beans showed an isomer ratio of 80:20 for trans/cis-JA. After wounding, de novo synthesis of JA alters the ratio to 20:80 in favor of the cis isomer.     

Cathepsins D and L reduce the toxicity of advanced glycation end products

Advanced glycation end product-modified proteins are known for accumulating during aging and in several pathological conditions such as diabetes, renal failure, and neurodegenerative disorders. There is little information about the intracellular fate of endocytosed advanced glycation end products (AGEs) and their influence on proteolytic systems. However, it is known that the lysosomal system is impaired during aging. Therefore, undegraded material may accumulate and play a considerable role in the development of diverse diseases. To investigate if AGEs can be degraded and to test whether they accumulate because of impaired lysosomal proteases we studied the effects of advanced glycation end products on the endosomal-lysosomal system. Five different types of AGEs were generated by bovine serum albumin incubation with glyoxal, methylglyoxal, glucose, fructose, and ribose. The first experiments revealed the uptake of AGEs by the macrophage cell line RAW 264.7. Further investigations demonstrated an increase in cathepsin D and L activity and an increase in mature cathepsins D and L. Increased activities were accompanied by the presence of more lysosomes, measured by staining with LysoTracker blue. To specify the roles of cathepsins D and L we used knockout cells to test the roles of both cathepsins on the toxicity of advanced glycation end products. In summary we conclude that both cathepsins are required for a reduction in advanced glycation end product-induced cytotoxicity.     

Exploring the genome sequence of Bifidobacterium bifidum S17 for potential players in host-microbe interactions

Bifidobacterium bifidum S17 is a promising probiotic candidate, which displays potent anti-inflammatory activity both in vitro and in vivo. Furthermore, tight adhesion to intestinal epithelial cells (IECs) has been shown for this strain and is considered to be an important prerequisite for host colonization and consequently the anti-inflammatory properties. By analyzing the recently sequenced genome at least 9 surface proteins and various gene clusters encoding for putative pili were identified. Most of these proteins are expressed in vitro, with higher expression during exponential growth phase. Increased expression of the putative adhesins in exponential growth phase was associated with higher adhesion of B. bifidum S17 to Caco-2 cells.