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Dieter Müller

Publications and source records attributed to Dieter Müller.

At least 19 recordsLinked to original sources
PubMed2006

Identification of apolipoprotein A-I as a "STOP" signal for myopia.

Good visual acuity requires that the axial length of the ocular globe is matched to the refractive power of the cornea and lens to focus the images of distant objects onto the retina. During the growth of the juvenile eye, this is achieved through the emmetropization process that adjusts the ocular axial length to compensate for the refractive changes that occur in the anterior segment. A failure of the emmetropization process can result in either excessive or insufficient axial growth, leading to myopia or hyperopia, respectively. Emmetropization is mainly regulated by the retina, which generates two opposite signals: "GO/GROW" signals to increase axial growth and "STOP" signals to block it. The presence of GO/GROW and STOP signals was investigated by a proteomics analysis of the retinas from chicken with experimental myopia and hyperopia. Of 18 differentially expressed proteins that were identified, five displayed an expression profile corresponding to GO/GROW signals, and two corresponded to STOP signals. Western blotting confirmed that apolipoprotein A-I (apoA-I) has the characteristics of a STOP signal both in the retina as well as in the fibrous sclera. In accordance with this, intraocular application of the peroxisome proliferator-activated receptor alpha agonist GW7647 resulted in up-regulation of apoA-I levels and in a significant reduction of experimental myopia. In conclusion, using a comprehensive functional proteomics analysis of chicken ocular growth models we identified targets for ocular growth control. The correlation of elevated apoA-I levels with reduced ocular axial growth points toward a functional relationship with the observed morphological changes of the eye.

Animals↗
PubMed2006

Influence of Verapamil and Cyclosporin A on bile acid metabolism and transport in rat liver slices.

Verapamil (V) is a specific inhibitor of the P-glycoprotein (mdr1) in the hepatocyte canalicular membrane. Cyclosporin A (CsA) as an essential immunosuppressive drug has potentially cholestatic adverse effects on the liver, but increases the expression of mdr1. In precision-cut liver slices from 34- to 40-day-old male Wistar rats 26 individual free and conjugated bile acids (BAs) as markers of hepatic transport and synthesis function were analysed after 4 h incubation with V (100 microM) or CsA (5 microM) in Krebs-Henseleit buffer. Some slices were loaded with cholic acid (CA 5 microM) or tauro-ursodeoxycholic acid (T-UDCA 5 microM) to investigate the V and CsA effects under conditions of BA supplementation. BAs were determined in tissue and medium by HPLC with postcolumn derivatisation and fluorescence detection. V and CsA, influencing different targets in BA transport, enhanced slice concentrations of T- and glyco- (G-) conjugated CA only when exogenous CA was given additionally. This BA accumulation in tissue is more reflected at decreased medium concentrations of these BAs after V and CsA incubations. Both V and CsA also inhibited CA uptake into the slices. The acidic chenodeoxycholic acid (CDCA) synthesis pathway is disturbed: T- and G-CDCA concentrations are diminished in slices and medium after V and CsA incubations. T-UDCA plus V or CsA enhanced not only its own slice concentration but also the concentration of the trihydroxylated tauro-muricholic acid (T-beta-MCA), reflecting the conversion of the accumulated dihydroxylated T-UDCA into the T-beta-MCA. The similar effects of V and CsA on BA transport and metabolism can be explained by mdr1 mediated disturbances of cellular ATP transport rather than by inhibition of individual BA transporters.

Animals↗
PubMed2006

Homologous and lysophosphatidic acid-induced desensitization of the atrial natriuretic peptide receptor, guanylyl cyclase-A, in MA-10 leydig cells.

The cardiac hormone atrial natriuretic peptide (ANP) signals via interaction with a plasma membrane receptor, which has guanylyl cyclase (GC) activity and is referred to as GC-A. Desensitization of GC-A is thought to represent a physiologically important regulatory mechanism, but the signaling pathways implicated and cell type-specific effects are still poorly understood. Here we demonstrate that sustained exposure to either ANP itself or the bioactive lipid lysophosphatidic acid (LPA) elicits GC-A desensitization in MA-10 Leydig cells. Both reactions show similar kinetics and evoke equal decreases (by 40%) in GC-A hormone responsiveness. Homologous (ANP induced) desensitization, in which cGMP is generated as second messenger, is blocked by distinct cAMP-dependent protein kinase [protein kinase A (PKA)] inhibitors, H 89, and Rp-8-CPT-cAMPs, providing evidence that PKA mediates the reaction. Accordingly, the ANP/cGMP-elicited effects are mimicked by a cAMP analog, 8-bromo-cAMP. The LPA-induced (heterologous) desensitization is not blocked by PKA inhibition, indicating a different signaling pathway. LPA, but not ANP, enhances ERK phosphorylation and induces cell rounding together with a dramatic reorganization of actin filaments. Consistent with the identification of LPA receptor (LPA2 and LPA3) gene expression, the findings are indicative of LPA receptor-mediated reactions. This study demonstrates for the first time coexistence of homologous and heterologous desensitization of GC-A in the same cell type, reveals that these reactions are mediated by different pathways, and identifies a novel cross talk between phospholipid and natriuretic peptide signaling. The morphoregulatory activities exerted by LPA suggest a crucial role for Leydig cell physiology.

Animals↗
PubMed2006

The expression of neurotrophins and their receptors in the prenatal and adult human testis: evidence for functions in Leydig cells.

Previous studies have demonstrated local functions for neurotrophins in the developing and mature testis of rodents. To examine whether these signaling molecules are present and also potentially active in the human testis, we characterized immunohistochemically the expression and cellular localization of the known neurotrophins and their receptors during prenatal testicular development as well as in the adult human testis. Results obtained revealed the presence of nerve growth factor (NGF), brain-derived neurotrophic factor, neurotrophin-3 and 4, as well as neurotrophin receptors p75(NTR), TrkA, TrkB, and TrkC during testis morphogenesis. These proteins were also detectable in the adult human testis, and their local expression could be confirmed largely by immunoblot and RT-PCR analyses. Remarkably, the Leydig cells were found to represent the predominant neurotrophin/receptor expression sites within both fetal and adult human testes. Functional assays performed with a mouse tumor Leydig cell line revealed that NGF exposure increases cellular steroid production, indicating a role in differentiation processes. These findings support previously-recognized neuronal characteristics of Leydig cells, provide additional evidence for potential roles of neurotrophins during testis morphogenesis and in the mature testis, and demonstrate for the first time a neurotrophin-induced functional activity in Leydig cells.

Aged↗
PubMed2006

Regulation of spontaneous contractile activity in the bovine epididymal duct by cyclic guanosine 5'-monophosphate-dependent pathways.

Passage of spermatozoa through the epididymis is obligatory for sperm maturation processes and is based on spontaneous phasic contractions (SC) of the epididymal duct. Here, the functional role of cyclic GMP (cGMP) signaling in modulating SC in the bovine epididymal caput and corpus region was examined by muscle tension recording and immunological and autoradiographic techniques. The cGMP-analog 8-bromo (Br)-cGMP, as well as the nitric oxide (NO) donor sodium nitroprusside and the natriuretic peptides (NPs) atrial NP and C-type NP, displayed distally increasing SC-relaxant effects. In agreement, a distally increasing epididymal expression of the cGMP-dependent protein kinase I (PKG I), endothelial NO synthase (eNOS), and the atrial NP receptor was found. Immunoreactivity for PKG, soluble guanylate cyclase, and eNOS could be localized to the epididymal muscle cells as well as to the epithelial basal cells only at the corpus level. The SC-relevant action of NO and the NPs was cGMP dependent, and the action of 8-Br-cGMP, in turn, was modified by epithelial and luminal factors. The NOS inhibitor L-NAME (N(omega)-nitro-L-arginine methyl ester) caused an increase in SC frequency, indicating basal activity of NO generating enzymes. The SC-inhibitory effect of 8-Br-cGMP was clearly reduced by the PKG inhibitor Rp-8-Br-cGMPS as well as by iberiotoxin, thapsigargin, and indomethacin, pointing to PKG as main SC-relevant target of cGMP, and to large-conductance calcium-activated K(+) channels, the sarcoplasmic-endoplasmic reticulum Ca(2+)-ATPase and cyclooxygenase-1 as possible targets of PKG. These data support an essential role of cGMP signaling in the control of epididymal peristalsis, thereby enabling fine tuning of sperm transport and maturation.

Animals↗
PubMed2006

Bile acid transport and metabolism in rat liver slices.

To further characterise precision-cut liver slices from 34- to 40-day-old male rats as an in vitro model for bile acid (BA) metabolism and transport, the effect of the primary BAs cholic (CA, 5 microM) and chenodeoxycholic acid (CDCA, 0.15 and 0.75 microM) as well as of the therapeutically used tauroursodeoxycholic acid (T-UDCA, 5 microM) on BA profiles was investigated. After 4 h incubation in 5 ml Krebs-Henseleit buffer (KHB) 26 individual BAs were determined in slices (50 mg liver/5 ml KHB) and medium by HPLC with postcolumn derivatisation and fluorescence detection. In control incubations, mean total BA concentrations were 5.09 nmol/50 mg liver (101.80 nmol/g liver) in slices and 25.71 nmol/5 ml KHB, among them 72% taurine-(T-), 22% glycine-(G-) conjugated and 6% free BAs in tissue and medium. The main BAs were beta-muricholic (beta-MCA and conjugates) and cholic acids (CA and conjugates) in tissue and medium. The following results were obtained after addition of CDCA, CA, and T-UDCA, respectively, to the KHB. The toxic CDCA was quantitatively converted mainly to T-UDCA and taurohyodeoxycholic (T-HDCA) acid. CA was conjugated in equal shares to T- and G-CA, whereas T-UDCA was enriched in slices and hydroxylated half to T-beta-MCA, which is the main BA in rats. In conclusion, rat liver slices are highly effective not only in uptake, conjugation and excretion of BAs but also in conversion of strong detergent into less toxic BAs.

Animals↗
PubMed2006

The ether lipid inositol-C2-PAF is a potent inhibitor of cell proliferation in HaCaT cells.

The search for specific anticancer drugs that do not interfere with DNA synthesis or influence the cytoskeleton has led to the development of modified phospholipids with antiproliferative properties. These compounds cause remodeling of the structure and function of plasma membranes. Recently, we described novel compounds, the glycosidated phospholipids, that surprisingly inhibit cell proliferation. These compounds contain alpha-D-glucose in the sn-2 position of the glycerol backbone of phosphatidylcholine (PC) and platelet-activating factor (PAF), which gives rise to 2-glucophosphatidylcholine (Glc-PC) and 1-O-octadecyl-2-O-alpha-d-glucopyranosyl-sn-2-glycero-3-phosphatidylcholine (Glc-PAF), respectively. Glc-PC and Glc-PAF inhibit the growth of HaCaT cells at nontoxic concentrations. Here we report the introduction of myo-inositol, in place of alpha-D-glucose, in the sn-2 position of the glycerol backbone; this leads to two diastereomeric 1-O-octadecyl-2-O-(2-(myo-inositolyl)-ethyl)-sn-glycero-3-(R/S)-phosphatidylcholines (Ino-C2-PAF). The inositol-containing PAF enhances the antiproliferative capacity (IC(50)=1.8 microM) and reduces the cytotoxicity relative to Glc-PAF (LC(50)=15 microM). Through biological assays, we showed that, in HaCaT cells, Ino-C2-PAF causes upregulation of the keratinocyte-specific differentiation marker involucrin, increases the activity of the differentiation marker transglutaminase, and induces apoptosis at nontoxic concentrations. Ino-C2-PAF therefore seems to be a promising candidate for development as an antiproliferative drug for the treatment of hyperproliferative diseases of the skin.

Cell Differentiation↗
PubMed2005

The garlic ingredient diallyl sulfide inhibits cytochrome P450 2E1 dependent bioactivation of acrylamide to glycidamide.

Genotoxic effects of acrylamide are supposed to result from oxidative biotransformation to glycidamide. After incubation of rat liver slices with acrylamide we detected free glycidamide using a liquid chromatography tandem mass spectrometric method. Glycidamide formation was diminished in the presence of the cytochrome P450 2E1 inhibitor diallyl sulfide (DAS), which is a specific ingredient of garlic. This may be relevant to human health since the suggested carcinogenic risk of dietary acrylamide may be reduced by concomitant intake of garlic.

Acrylamide↗
PubMed2005

Catecholamine-synthesizing enzymes in the adult and prenatal human testis.

Catecholamines play functional roles in the mature and developing mammalian testis but the cell types responsible for their local synthesis are still controversially discussed. Here, we demonstrate that four enzymes involved in the biosynthesis of catecholamines, namely, tyrosine hydroxylase (TH), aromatic amino acid decarboxylase (AADC), dopamine beta-hydroxylase (DBH) and phenylethanolamine- N-methyltransferase (PNMT), are expressed in Leydig cells of the human testis. Tyrosine hydroxylase, the key enzyme of the biosynthesis of catecholamines, was localized to Leydig cells both at the transcript level (by RT-PCR analyses and by in situ hybridization assays) and at the protein level (by immunoblotting and by immunohistochemistry). The other enzymes were also demonstrated in Leydig cells by RT-PCR and immunohistochemical analyses. The presence of TH, AADC, DBH, and PNMT in human Leydig cells was found, in addition, by immunohistochemical approaches carried out on sections from prenatal human testes. Thus, the present study identifies the Leydig cells as the presumed sites of catecholamine production in both the mature and fetal human testes and further supports the previously recognized neuroendocrine characteristics of this cell type.

Aged↗
PubMed2005

Histomorphological changes and cytochrome P450 isoforms expression and activities in precision-cut liver slices from neonatal rats.

Precision-cut rat liver slices are a widely accepted in vitro tool for the examination of drug metabolism, enzyme induction or hepatotoxic effects of xenobiotics. After prolonged incubation, however, distinct histopathological changes and increasing losses in function are seen with liver slices from adult animals. Since tissue from neonatal animals is expected to be less vulnerable, in the present study liver slices from 1-day-old rats were examined for morphological changes and for the expression of different cytochrome P450 (CYP) isoforms after incubation for up to 24 h and after a 24 h in vitro exposure to beta-naphthoflavone (BNF), phenobarbital (PB), dexamethasone (DEX) or pregnenolone 16alpha-carbonitrile (PCN). In parallel, CYP activities were assessed by different model reactions in slice homogenates and in intact slices. Histopathological changes were less pronounced in liver slices from 1-day-old rats than in those from adult animals. During the 24 h of incubation even a maturation of the tissue occurred, since the proportion of haemopoietic stem cells declined and the glycogen content of the hepatocytes increased. The CYP expression pattern after 2 and 24 h of incubation was similar to that of normal liver specimens from neonatal rats showing a moderate CYP1A1, 2B1 and 3A2 expression. The immunostaining for CYP1A1 and 2B1 was elevated after incubation with BNF. PB enhanced CYP2B1 and 3A2 expression, and DEX and PCN increased CYP3A2 immunostaining. This induction pattern was paralleled by respective effects on the corresponding model reactions. Thus, besides increased viability, slices from neonatal rats are excellently suited for the evaluation of an in vitro induction of CYP enzymes as well.

Animals↗
PubMed2004

Progenitor cells of the testosterone-producing Leydig cells revealed.

The cells responsible for production of the male sex hormone testosterone, the Leydig cells of the testis, are post-mitotic cells with neuroendocrine characteristics. Their origin during ontogeny and regeneration processes is still a matter of debate. Here, we show that cells of testicular blood vessels, namely vascular smooth muscle cells and pericytes, are the progenitors of Leydig cells. Resembling stem cells of the nervous system, the Leydig cell progenitors are characterized by the expression of nestin. Using an in vivo model to induce and monitor the synchronized generation of a completely new Leydig cell population in adult rats, we demonstrate specific proliferation of vascular progenitors and their subsequent transdifferentiation into steroidogenic Leydig cells which, in addition, rapidly acquire neuronal and glial properties. These findings, shown to be representative also for ontogenetic Leydig cell formation and for the human testis, provide further evidence that cellular components of blood vessels can act as progenitor cells for organogenesis and repair.

Animals↗
PubMed2004

Influence of recipient gender on intrasplenic fetal liver tissue transplants in rats: cytochrome P450-mediated monooxygenase functions.

Rat livers display a sex-specific cytochrome P450 (P450) isoforms expression pattern with consecutive differences in P450-mediated monooxygenase activities, which have been shown to be due to a differential profile of growth hormone (GH) secretion. Parallel to previous investigations on P450 isoforms expression, the aim of the present study was to elucidate the influence of recipient gender on P450-mediated monooxygenase activities in intrasplenic liver tissue transplants in comparison to orthotopic liver. Fetal liver tissue suspensions of mixed gender were transplanted into the spleen of adult male or female syngenic recipients. Four months after grafting transplant-recipients and age-matched controls were treated with beta-naphthoflavone (BNF), phenobarbital (PB), dexamethasone (DEX) or the vehicles and sacrificed 24 or 48 h thereafter. P450-dependent monooxygenase activities were assessed by a series of model reactions for different P450 subtypes in liver and spleen 9000 g supernatants. In spleens of male and female control rats only very low monooxygenase activities were detectable, whereas with most model reactions distinct activities were observed in transplant-containing organs. Livers and transplant-containing spleens from male rats displayed higher basal ethoxycoumarin O-deethylase and testosterone 2alpha-, 2beta-, 6beta-, 14alpha-, 15alpha-, 15beta-, 16alpha-, 16beta- and 17-hydroxylase activities than those from females. On the other hand, like the respective livers, spleens from female transplant-recipients demonstrated more pronounced p-nitrophenol- and testosterone 6alpha- and 7alpha-hydroxylase activities than those from male hosts. With nearly all model reactions gender-specific differences in inducibility by BNF, PB or DEX could be demonstrated in livers as well as in transplant-containing spleens. These results further confirm that the P450 system of intrasplenic liver tissue transplants and the respective orthotopic livers is similarly influenced by recipient gender.

Animals↗
PubMed2004

Comparative analysis between cyclic GMP and cyclic AMP signalling in human sperm.

The principal involvement of cyclic nucleotides in regulating sperm functions is well established, but the factors controlling their generation and actions have not yet been entirely resolved. In particular, specific roles for cyclic (c)GMP in mammalian sperm are poorly understood. In this study, we have characterized comparatively the cAMP and cGMP signalling systems in ejaculated human sperm. Mean concentrations of cGMP (0.1 micromol/l) were found to be 100-fold lower than those of cAMP in non-stimulated cells, and adenylyl cyclase (AC) activities predominate by far guanylyl cyclase (GC) activities in both particulate and soluble protein fractions. By different experimental approaches (photoaffinity labelling, cyclase assays, immunoblotting), we provide evidence for the presence (guanylyl cyclase-A, soluble guanylyl cyclase, regulatory and catalytic subunits of cAMP-dependent protein kinase) or absence (guanylyl cyclase-B, natriuretic peptide clearance receptor, neuronal nitric oxide synthase, cGMP-dependent protein kinase I) of different factors involved in either cAMP or cGMP pathways. Functional studies showed that cGMP, at high concentrations, can enhance sperm protein tyrosine phosphorylation but not serine phosphorylation of glycogen synthase kinase. This study reveals that human sperm are characterized by an exceptional predominance of cAMP signalling and indicates potential roles for cGMP.

Adenylyl Cyclases↗
PubMed2004

Most effective stimulation site in subthalamic deep brain stimulation for Parkinson's disease.

The optimal stimulation site in subthalamic deep brain stimulation (STN-DBS) was evaluated by correlation of the stereotactic position of the stimulation electrode with the electrophysiologically specified dorsal STN border. In a series of 25 electrodes, best clinical results with least energy consumption were found in contacts located in the dorsolateral border zone, whereas contacts within the subthalamic white matter, e.g., zona incerta, were significantly less effective. We suggest that the dorsolateral STN border should be covered by STN-DBS.

Electric Stimulation Therapy↗
PubMed2003

Spatiotemporal regulation of the two atrial natriuretic peptide receptors in testis.

By interacting with a guanylyl cyclase (GC) activity-containing receptor, termed GC-A, atrial natriuretic peptide (ANP) acts as a regulator of blood pressure and fluid volume homeostasis. High expression levels of GC-A in the testis and reported effects of ANP on testosterone secretion by Leydig cells are indicative of important local functions in this organ. Here we show, based on radioligand receptor labeling and immunological approaches, that seminiferous tubules rather than Leydig cells are the predominant GC-A expression sites in the rat testis. Functional activity was proved by ANP- induced cGMP accumulation in isolated seminiferous tubules. Although ontogenetic studies revealed a massive increase in GC-A levels during sexual maturation, the so-called natriuretic peptide clearance receptor, another type of ANP receptor proposed to locally control the availability of natriuretic peptides, was found to be expressed predominantly before puberty, exceeding the level of GC-A expression at this time. Natriuretic peptide clearance receptor also shows a distinct distribution pattern surrounding the seminiferous tubules. These findings raise the possibility of novel physiological roles for ANP and cGMP in the testis related to germ cell maturation and/or the regulation of the onset of puberty and suggest that the two ANP receptors function in a coordinated manner at this target organ.

Animals↗
PubMed2003

Influence of recipient gender on cytochrome P450 isoforms expression in intrasplenic fetal liver tissue transplants in rats.

Rat livers display a sex-specific cytochrome P450 (P450) isoforms expression pattern which is regulated by a differential profile of growth hormone (GH) secretion. The aim of the present study was to elucidate whether liver cell transplants at an ectopic site are also subject to this influence. Fetal liver tissue suspensions of mixed gender were transplanted into the spleen of adult male or female syngenic recipients. Four months after grafting transplant recipients and age-matched controls were treated with beta-naphthoflavone (BNF), phenobarbital (PB), dexamethasone (DEX) or the solvents and sacrificed 24 or 48 h thereafter. Livers and intrasplenic transplants were evaluated for the expression of the P450 subtypes 1A1, 2B1, 2E1, 3A2 and 4A1 by means of immunohistochemistry. The livers of both male and female rats displayed nearly no P450 1A1, but a distinct P450 2B1, 2E1, 3A2 and 4A1 expression. Whereas no sex differences were seen in the P450 1A1 expression, the immunostaining for P450 2B1, 3A2 and 4A1 was stronger in males and that for P450 2E1 in females. Similarly, in the intrasplenic liver cell transplants almost no P450 1A1, but a noticeable P450 2B1, 2E1, 3A2 and 4A1 expression was observed. Like in the respective livers, the immunostaining for P450 2B1, 3A2 and 4A1 was stronger in the transplants hosted by male than by female rats, whereas the opposite was the case for the P450 2E1 expression. Both in livers and transplants with some sex-specific differences P450 1A1 and 2E1 expression was induced by BNF, that of P450 2B1 by BNF and PB, and that of P450 3A2 by PB and DEX. These results indicate that the P450 system of ectopically transplanted liver cells is influenced by the gender of the recipient organism like that of the orthotopic livers.

Animals↗
PubMed2003

Non-covalent interaction of ubiquitin with insulin-degrading enzyme.

Insulin-degrading enzyme (IDE) is a metalloprotease implicated in insulin degradation and suggested to have a variety of additional functions, including the clearance of amyloid beta peptides of Alzheimer's disease. Little is known about endogenous proteins that may interact with and modulate IDE's activity in the cell. We purified and characterized two proteins from mouse leukemic splenocytes that interact with IDE and inhibit its insulin-degrading activity. A protein of 14 kDa was similar to a competitive IDE inhibitor reported previously. The major inhibitor was identified by amino acid sequencing as ubiquitin, a protein that is post-translationally covalently attached to other intracellular proteins and regulates diverse cellular processes. Ubiquitin inhibited insulin-degrading activity of IDE and diminished crosslinking of 125I-insulin to IDE in a specific, concentration-dependent, reversible, and ATP-independent manner. Ubiquitin did not affect the crosslinking of 125I-insulin to insulin receptors or of 125I-atrial natriuretic peptide (ANP) to its receptor guanylate cyclase-A. These findings suggest a novel role for ubiquitin or perhaps proteins with ubiquitin-like domains in regulating the function of IDE.

Animals↗