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Biomedical subjects

Andreas Ludwig

Publications and source records attributed to Andreas Ludwig.

35 records · Page 2Linked to original sources
PubMed2004

Melanocyte-specific expression of dopachrome tautomerase is dependent on synergistic gene activation by the Sox10 and Mitf transcription factors.

Sox10 regulates melanocyte development at least partly through its stimulatory effect on Mitf gene expression. Here, we characterize the gene for dopachrome tautomerase (Dct/Trp2) as the second direct Sox10 target in melanocytes, arguing for the existence of Sox10 functions in melanocytes that are independent of its epistatic relationship to Mitf. Sox10 responsiveness was mediated by multiple binding sites within the proximal Dct/Trp2 promoter which display varying affinities and bind Sox10 monomers or dimers. Mitf synergistically enhanced Sox10-dependent activation of the Dct/Trp2 promoter. Synergy appears mechanistically complex and requires both direct binding of Sox10 to the promoter and the protein's transactivation domain.

Animals↗
PubMed2004

Sox10-rtTA mouse line for tetracycline-inducible expression of transgenes in neural crest cells and oligodendrocytes.

Using gene targeting, we inserted a high-affinity variant of the reverse tetracycline controlled transactivator (rtTA) into the genomic Sox10 locus. This rtTA transgene faithfully recapitulated Sox10 expression in the emerging neural crest, several of its derivatives, and in oligodendrocytes. It was furthermore able to induce expression of a tetracycline inducible transgenic reporter gene in a doxycycline-dependent manner. Induction was fast, with substantial reporter gene expression visible 6 h after the onset of doxycycline treatment. Shut-off, in contrast, exhibited delayed kinetics, which probably correlated with doxycycline clearance rates. This mouse provides a useful tool for generating tetracycline-controlled gene expression in neural crest and oligodendrocytes.

Animals↗
PubMed2004

Pacemaker channels and sinus node arrhythmia.

Cardiac pacemaker activity is regulated by at least five different classes of ion channels and by the opposing influence of sympathetic and parasympathetic stimulation. Inactivation of several genes, including a subunit coding for the potassium channel activated by the muscarinic receptor, I(KACh); the calcium channel, I(Ca,); and the hyperpolarization-activated channel, I(f), results in sinus node arrhythmia. Inactivation of the gene for the hyperpolarization-activated, cyclic nucleotide-gated channel isoform HCN2 or HCN4 and the use of pacemaker channel blockers show that (a) HCN2 prevents the diastolic membrane potential from becoming too negative, (b) HCN4 is the major channel mediating sympathetic stimulation of the pacemaker activity, and (3) complete blockage of the I(f) current is compatible with slow sinus node rhythm.

Action Potentials↗
PubMed2004

Different proliferative activity of the glandular and myoepithelial lineages in benign proliferative and early malignant breast diseases.

The aim of the present study was to explore cell biological characteristics of normal breast, benign proliferative breast diseases and noninvasive breast malignancies based on the recently published adult progenitor cell concept from our group. Here, we investigated the proliferative activity of CK5/14(+), CK8/18/19(+) and alpha-smooth muscle actin(+) cellular phenotypes encountered in normal mammary gland, in a series of usual ductal hyperplasias and early malignant breast diseases, such as atypical ductal and lobular hyperplasias, as well as ductal and lobular in situ carcinomas. Immunohistochemical double labeling was performed on frozen sections from diagnostic breast biopsies by using antibodies to basal cytokeratins (CK5/14), glandular cytokeratins (CK8/18/19), smooth muscle actin and the Ki-67 antigen (MIB1). Normal breast tissues and usual ductal hyperplasias were characterized by a heterogeneous cellular composition of the growth fraction. The proliferative cell compartment consisted of CK8/18/19(+) glandular and, in a variable proportion, CK5/14(+) progenitor phenotypes. In contrast, noninvasive breast malignancies were composed of a monotonous proliferation of CK 8/18/19(+) neoplastic glandular cells. These findings indicate a significant role of progenitor cells in the development of benign proliferative breast diseases and lend support to the view that malignant transformation in the human breast usually occurs in a cell committed to the glandular lineage. Our results provide cell kinetic support to the functional progenitor cell hypothesis, and we propose this concept as an operative model for understanding benign proliferative and malignant breast diseases.

Actins↗
PubMed2004

AtSUC8 and AtSUC9 encode functional sucrose transporters, but the closely related AtSUC6 and AtSUC7 genes encode aberrant proteins in different Arabidopsis ecotypes.

Three members of the Arabidopsis sucrose transporter gene family, AtSUC6-AtSUC8 (At5g43610; At1g66570; At2g14670), share a high degree of sequence homology in their coding regions and even in their introns and in their 5'- and 3'-flanking regions. A fourth sucrose transporter gene, AtSUC9 (At5g06170), which is on the same branch of the AtSUC-phylogenetic tree, shows only slightly less sequence homology. Here we present data demonstrating that two genes from this subgroup, AtSUC6 and AtSUC7, encode aberrant proteins and seem to represent sucrose transporter pseudogenes, whereas AtSUC8 and AtSUC9 encode functional sucrose transporters. These results are based on analyses of splice patterns and polymorphic sites between these genes in different Arabidopsis ecotypes, as well as on functional analyses by cDNA expression in baker's yeast. For one of these genes, AtSUC7 (At1g66570), different, ecotype-specific splice patterns were observed in Wassilewskija (Ws), C24, Columbia wild type (Col-0) and Landsberg erecta (Ler). No incorrect splicing and no sequence polymorphism were detected in the cDNAs of AtSUC8 and AtSUC9, which encode functional sucrose transporters and are expressed in floral tissue. Finally, promoter-reporter gene plants and T-DNA insertion lines were analyzed for AtSUC8 and AtSUC9.

Amino Acid Sequence↗
PubMed2003

The hyperpolarization-activated channel HCN4 is required for the generation of pacemaker action potentials in the embryonic heart.

Hyperpolarization-activated, cyclic nucleotide-gated cation currents, termed If or Ih, are generated by four members of the hyperpolarization-activated, cyclic nucleotide-gated cation (HCN) channel family. These currents have been proposed to contribute to several functions including pacemaker activity in heart and brain, control of resting potential, and neuronal plasticity. Transcripts of the HCN4 isoform have been found in cardiomyocytes and neurons, but the physiological role of this channel is unknown. Here we show that HCN4 is essential for the proper function of the developing cardiac conduction system. In wild-type embryos, HCN4 is highly expressed in the cardiac region where the early sinoatrial node develops. Mice lacking HCN4 channels globally, as well as mice with a selective deletion of HCN4 in cardiomyocytes, died between embryonic days 9.5 and 11.5. On average, If in cardiomyocytes from mutant embryos is reduced by 85%. Hearts from HCN4-deficient embryos contracted significantly slower compared with wild type and could not be stimulated by cAMP. In both wild-type and HCN4-/- mice, cardiac cells with "primitive" pacemaker action potentials could be found. However, cardiac cells with "mature" pacemaker potentials, observed in wild-type embryos starting at day 9.0, were not detected in HCN4-deficient embryos. Thus, HCN4 channels are essential for the proper generation of pacemaker potentials in the emerging sinoatrial node.

Action Potentials↗
PubMed2003

Role of subunit heteromerization and N-linked glycosylation in the formation of functional hyperpolarization-activated cyclic nucleotide-gated channels.

The coassembly of homologous subunits to heteromeric complexes serves as an important mechanism in generating ion channel diversity. Here, we have studied heteromerization in the hyperpolarization-activated cyclic nucleotide-gated (HCN) channel family. Using a combination of fluorescence confocal microscopy, coimmunoprecipitation, and electrophysiology we found that upon coexpression in HEK293 cells almost all dimeric combinations of HCN channel subunits give rise to the formation of stable channel complexes in the plasma membrane. We also identified HCN1/HCN2 heteromers in mouse brain indicating that heteromeric channels exist in vivo. Surprisingly, HCN2 and HCN3 did not coassemble to heteromeric channels. This finding indicates that heteromerization requires specific structural determinants that are not present in all HCN channel combinations. Using N-glycosidase F we show that native as well as recombinant HCN channels are glycosylated resulting in a 10-20-kDa shift in the molecular weight. Tunicamycin, an inhibitor of N-linked glycosylation, blocked surface membrane expression of HCN2. Similarly, a mutant HCN2 channel in which the putative N-glycosylation site in the loop between S5 and the pore helix was replaced by glutamine (HCN2N380Q) was not inserted into the plasma membrane and did not yield detectable whole-cell currents. These results indicate that N-linked glycosylation is required for cell surface trafficking of HCN channels. Cotransfection of HCN2N380Q with HCN4, but not with HCN3, rescued cell surface expression of HCN2N380Q. Immunoprecipitation revealed that this rescue was due to the formation of a HCN2N380Q/HCN4 heteromeric channel. Taken together our results indicate that subunit heteromerization and glycosylation are important determinants of the formation of native HCN channels.

Animals↗
PubMed2003

Cellular cholesterol depletion triggers shedding of the human interleukin-6 receptor by ADAM10 and ADAM17 (TACE).

Interleukin-6 (IL-6) activates cells by binding to the membrane-bound IL-6 receptor (IL-6R) and subsequent formation of a glycoprotein 130 homodimer. Cells that express glycoprotein 130, but not the IL-6R, can be activated by IL-6 and the soluble IL-6R which is generated by shedding from the cell surface or by alternative splicing. Here we show that cholesterol depletion of cells with methyl-beta-cyclodextrin increases IL-6R shedding independent of protein kinase C activation and thus differs from phorbol ester-induced shedding. Contrary to cholesterol depletion, cholesterol enrichment did not increase IL-6R shedding. Shedding of the IL-6R because of cholesterol depletion is highly dependent on the metalloproteinase ADAM17 (tumor necrosis factor-alpha-converting enzyme), and the related ADAM10, which is identified here for the first time as an enzyme involved in constitutive and induced shedding of the human IL-6R. When combined with protein kinase C inhibition by staurosporine or rottlerin, breakdown of plasma membrane sphingomyelin or enrichment of the plasma membrane with ceramide also increased IL-6R shedding. The effect of cholesterol depletion was confirmed in human THP-1 and Hep3B cells and in primary human peripheral blood monocytes, which naturally express the IL-6R. For decades, high cholesterol levels have been considered harmful. This study indicates that low cholesterol levels may play a role in shedding of the membrane-bound IL-6R and thereby in the immunopathogenesis of human diseases.

ADAM Proteins↗
PubMed2003

The disintegrin-like metalloproteinase ADAM10 is involved in constitutive cleavage of CX3CL1 (fractalkine) and regulates CX3CL1-mediated cell-cell adhesion.

The CX3C chemokine fractalkine (CX3CL1) exists as a membrane-expressed protein promoting cell-cell adhesion and as a soluble molecule inducing chemotaxis. Transmembrane CX3CL1 is converted into its soluble form by defined proteolytic cleavage (shedding), which can be enhanced by stimulation with phorbol-12-myristate-13-acetate (PMA). PMA-induced CX3CL1 shedding has been shown to involve the tumor necrosis factor-alpha-converting enzyme (TACE), whereas the constitutive cleavage in unstimulated cells remains elusive. Here we demonstrate a role of the closely related disintegrin-like metalloproteinase 10 (ADAM10) in the constitutive CX3CL1 cleavage. The hydroxamate GW280264X, capable of blocking TACE as well as ADAM10, proved to be an effective inhibitor of the constitutive and the PMA-inducible CX3CL1 cleavage in CX3CL1-expressing ECV-304 cells (CX3CL1-ECV-304), whereas GI254023X, preferentially blocking ADAM10 but not TACE, reduced the constitutive cleavage only. Overexpression of ADAM10 in COS-7 cells enhanced constitutive cleavage of CX3CL1 and, more importantly, in murine fibroblasts deficient of ADAM10 constitutive CX3CL1 cleavage was markedly reduced. Thus, ADAM10 contributes to the constitutive shedding of CX3CL1 in unstimulated cells. Addressing the functional role of CX3CL1 shedding for the adhesion of monocytic cells via membrane-expressed CX3CL1, we found that THP-1 cells adhere to CX3CL1-ECV-304 cells but detach in the course of vigorous washing. Inhibition of ADAM10-mediated CX3CL1 shedding not only increased adhesive properties of CX3CL1-ECV-304 cells but also prevented de-adhesion of bound THP-1 cells. Our data demonstrate that ADAM10 is involved in the constitutive cleavage of CX3CL1 and thereby may regulate the recruitment of monocytic cells to CX3CL1-expressing cell layers.

ADAM Proteins↗
PubMed2003

Dominant-negative suppression of HCN channels markedly reduces the native pacemaker current I(f) and undermines spontaneous beating of neonatal cardiomyocytes.

BACKGROUND: The pacemaker current I(f) contributes to spontaneous diastolic depolarization of cardiac autonomic cells. In heterologous expression, HCN channels exhibit a hyperpolarization-activated inward current similar to I(f). However, the links between HCN genes and native I(f) are largely inferential, and it remains unknown whether I(f) is essential for cardiac pacing. METHODS AND RESULTS: To clarify this situation, we generated a GYG(402-404)AYA pore mutation of HCN2, which rendered the channel nonfunctional and suppressed wild-type HCN2 in a dominant-negative manner in Chinese hamster ovary cells. In addition, HCN2-AYA suppressed I(HCN4) in a dominant-negative manner when coexpressed with wild-type HCN4, indicating that the 2 isoforms HCN2 and HCN4 are able to coassemble to form heteromultimeric complexes. Given that HCN2 and HCN4 are the dominant HCN mRNA transcripts in neonatal rat ventricle, we expressed HCN2-AYA in neonatal cardiocytes using adenoviral gene transfer to test the effect of HCN suppression on native I(f). I(f) density was indeed reduced markedly, from 7.8+/-1.6 pA/pF (n=13) in control cells to 0.3+/-0.2 pA/pF (n=11) in HCN2-AYA-infected cells when measured at -130 mV (P<0.001). To probe the effect of HCN on cardiac pacing, we infected spontaneously beating neonatal monolayers with adenoviral vectors expressing wild-type and mutant HCN channels. Infection with HCN2 and HCN4 accelerated the beating rate significantly, to 230.5+/-8.6 bpm (n=12) and 223.5+/-12.3 bpm (n=10), respectively, compared with control cultures (83.4+/-4.5 bpm, n=13, P<0.001). Conversely, HCN2-AYA completely undermined spontaneous pacing of neonatal cardiocytes. CONCLUSIONS: HCN channels are the major molecular component of native I(f) and are critical for spontaneous beating of neonatal cardiomyocytes.

Adenoviridae↗
PubMed2003

Absence epilepsy and sinus dysrhythmia in mice lacking the pacemaker channel HCN2.

Hyperpolarization-activated cation (HCN) channels are believed to be involved in the generation of cardiac pacemaker depolarizations as well as in the control of neuronal excitability and plasticity. The contributions of the four individual HCN channel isoforms (HCN1-4) to these diverse functions are not known. Here we show that HCN2-deficient mice exhibit spontaneous absence seizures. The thalamocortical relay neurons of these mice displayed a near complete loss of the HCN current, resulting in a pronounced hyperpolarizing shift of the resting membrane potential, an altered response to depolarizing inputs and an increased susceptibility for oscillations. HCN2-null mice also displayed cardiac sinus dysrhythmia, a reduction of the sinoatrial HCN current and a shift of the maximum diastolic potential to hyperpolarized values. Mice with cardiomyocyte- specific deletion of HCN2 displayed the same dysrhythmia as mice lacking HCN2 globally, indicating that the dysrhythmia is indeed caused by sinoatrial dysfunction. Our results define the physiological role of the HCN2 subunit as a major determinant of membrane resting potential that is required for regular cardiac and neuronal rhythmicity.

Animals↗
PubMed2002

Cooperative binding of Sox10 to DNA: requirements and consequences.

The high-mobility-group (HMG) domain containing transcription factor Sox10 is an important regulator of various processes including the development of neural crest cells and glial cells. Target gene promoters contain multiple Sox10-binding sites, which either support monomeric or cooperative, dimeric binding. The latter is unusual for Sox proteins and might contribute to functional specificity of Sox10. We find that specific amino acid residues in a conserved region immediately preceding the HMG domain of Sox10 are required for cooperative binding. These residues cooperate with the HMG domain during dimeric binding in a manner dependent on specific determinants within the first two alpha-helices of the HMG domain. Cooperativity of DNA binding is surprisingly refractory to changes in the overall conformation of the DNA-bound dimer. Whereas maintenance of cooperativity is essential for full activation of the promoter of the myelin protein zero target gene, dimer-dependent conformational changes such as the exact bending angle introduced into the promoter appear to be less important, shedding new light on the architectural function of Sox proteins.

Amino Acid Sequence↗
PubMed2002

Pain perception in mice lacking the beta3 subunit of voltage-activated calcium channels.

The importance of voltage-activated calcium channels in pain processing has been suggested by the spinal antinociceptive action of blockers of N- and P/Q-type calcium channels as well as by gene targeting of the alpha1B subunit (N-type). The accessory beta3 subunits of calcium channels are preferentially associated with the alpha1B subunit in neurones. Here we show that deletion of the beta3 subunit by gene targeting affects strongly the pain processing of mutant mice. We pinpoint this defect in the pain-related behavior and ascending pain pathways of the spinal cord in vivo and at the level of calcium channel currents and proteins in single dorsal root ganglion neurones in vitro. The pain induced by chemical inflammation is preferentially damped by deletion of beta3 subunits, whereas responses to acute thermal and mechanical harmful stimuli are reduced moderately or not at all, respectively. The defect results in a weak wind-up of spinal cord activity during intense afferent nerve stimulation. The molecular mechanism responsible for the phenotype was traced to low expression of N-type calcium channels (alpha1B) and functional alterations of calcium channel currents in neurones projecting to the spinal cord.

Amino Acid Sequence↗
PubMed2002

Fractalkine is expressed by smooth muscle cells in response to IFN-gamma and TNF-alpha and is modulated by metalloproteinase activity.

Fractalkine/CX3C-chemokine ligand 1 is expressed as a membrane-spanning adhesion molecule that can be cleaved from the cell surface to produce a soluble chemoattractant. Within the vasculature, fractalkine is known to be generated by endothelial cells, but to date there are no reports describing its expression by smooth muscle cells (SMC). In this study we demonstrate that IFN-gamma and TNF-alpha, but not IL-1beta, cooperate synergistically to induce fractalkine mRNA and protein expression in cultured aortic SMC. We also report the release of functional, soluble fractalkine from the membranes of stimulated SMC. This release is inhibited by the zinc metalloproteinase inhibitor batimastat, resulting in the accumulation of membrane-associated fractalkine on the SMC surface. Therefore, an SMC-derived metalloproteinase activity is involved in fractalkine shedding. While soluble fractalkine present in SMC-conditioned medium is capable of inducing calcium transients in cells expressing the fractalkine receptor (CX3CR1), blocking experiments using neutralizing Abs reveal that it can be inactivated without affecting the chemotactic activity of SMC-conditioned media on monocytes. However, membrane-bound fractalkine plays a major role in promoting adhesion of monocytic cells to activated SMC. This fractalkine-mediated adhesion is further enhanced in the presence of batimastat, indicating that shedding of fractalkine from the cell surface down-regulates the adhesive properties of SMC. Hence, during vascular inflammation, the synergistic induction of fractalkine by IFN-gamma and TNF-alpha together with its metalloproteinase-mediated cleavage may finely control the recruitment of monocytes to SMC within the blood vessel wall.

Cell Adhesion↗
PubMed2002

The CXC chemokine NAP-2 mediates differential heterologous desensitization of neutrophil effector functions elicited by platelet-activating factor.

During early inflammation, the chemoattractants neutrophil-activating peptide-2 (NAP-2), platelet-activating factor (PAF), and complement component C5a are rapidly generated within the vasculature and potently induce effector functions in neutrophils, such as chemotaxis and degranulation. We investigated whether these mediators would cross-desensitize each other's activities on isolated neutrophils. We demonstrate that NAP-2 and C5a desensitize degranulation of neutrophils in response to PAF. However, whereas C5a-mediated desensitization correlated with the downregulation of PAF binding sites on neutrophils, NAP-2 did not regulate PAF receptors, nor did it modulate their calcium signaling. In further contrast to C5a, which desensitized PAF-induced neutrophil chemotaxis, NAP-2 did not affect the chemotatic response to PAF. These observations indicate that NAP-2 mediates selective desensitization of PAF-induced neutrophil degranulation by a mechanism downstream from PAF receptors, still allowing migration toward PAF. Thus, a role for NAP-2 may be to limit PAF-dependent vascular tissue damage by preventing degranulation of neutrophils without affecting their migration into the inflamed tissue.

Blood Coagulation Factors↗
PubMed2002

Dipeptidyl peptidase IV (CD26) on T cells cleaves the CXC chemokine CXCL11 (I-TAC) and abolishes the stimulating but not the desensitizing potential of the chemokine.

Dipeptidyl peptidase IV (DPP IV/CD26) is a costimulatory molecule as well as a protease highly expressed on T cells. Purified DPP IV has been recognized to inactivate peptide hormones, neuropeptides, and some chemokines by cleavage behind a proline residue at the penultimate N-terminal amino acid position. Here, we identified another substrate for DPP IV among the chemokine family: the interferon-inducible T cell alpha chemoattractant (I-TAC/CXCL11). Using a specific DPP IV inhibitor, we demonstrate that DPP IV is responsible for the cleavage of the chemokine by PHA/IL-2-treated T cells. As PHA/IL-2-treated T cells also express the CXCL11 receptor (CXCR3), we investigated whether truncation of CXCL11 would modulate its biological activity for these cells. Truncated CXCL11 [CXCL11(3-73)] had an eightfold reduced potential to bind and to regulate CXCR3, but was completely inactive in calcium flux and chemotaxis assays. However, consistent with its reduced but still considerable ability to down-regulate CXCR3, truncated CXCL11 desensitized T cell chemotaxis in response to the intact chemokine. Hence, CXCL11-induced T cell recruitment may be regulated by DPP IV-mediated proteolytic inactivation of CXCL11 and furthermore by desensitization of T cells via the degradation product CXCL11(3-73).

Calcium↗