Veit Witzemann

Publications of Veit Witzemann

• A new mouse model for the slow-channel congenital myasthenic syndrome induced by the AChR εL221F mutation.

  Authors: Frédéric Chevessier, Christoph Peter, Ulrike Mersdorf, Emmanuelle Girard, Eric Krejci, Joseph J McArdle, Veit Witzemann

  Neurobiology of disease. 12/2011; 45(3):851-61.

  We have generated a new mouse model for congenital myasthenic syndromes by inserting the missense mutation L221F into the ε subunit of the acetylcholine receptor by homologous recombination. ThisWe have generated a new mouse model for congenital myasthenic syndromes by inserting the missense mutation L221F into the ε subunit of the acetylcholine receptor by homologous recombination. This mutation has been identified in man to cause a mild form of slow-channel congenital myasthenic syndrome with variable penetrance. In our mouse model we observe as in human patients prolonged endplate currents. The summation of endplate potentials may account for a depolarization block at increasing stimulus frequencies, moderate reduced muscle strength and tetanic fade. Calcium and intracellular vesicle accumulation as well as junctional fold loss and organelle degeneration underlying a typical endplate myopathy, were identified. Moreover, a remodeling of neuromuscular junctions occurs in a muscle-dependent pattern expressing variable phenotypic effects. Altogether, this mouse model provides new insight into the pathophysiology of congenital myasthenia and serves as a new tool for deciphering signaling pathways induced by excitotoxicity at peripheral synapses.

• Novel mouse model reveals distinct activity-dependent and -independent contributions to synapse development.

  Authors: Pier Giorgio Pacifici, Christoph Peter, Pessah Yampolsky, Michael Koenen, Joseph J McArdle, Veit Witzemann

  PloS one. 01/2011; 6(1):e16469.

  The balanced action of both pre- and postsynaptic organizers regulates the formation of neuromuscular junctions (NMJ). The precise mechanisms that control the regional specialization of acetylcholineThe balanced action of both pre- and postsynaptic organizers regulates the formation of neuromuscular junctions (NMJ). The precise mechanisms that control the regional specialization of acetylcholine receptor (AChR) aggregation, guide ingrowing axons and contribute to correct synaptic patterning are unknown. Synaptic activity is of central importance and to understand synaptogenesis, it is necessary to distinguish between activity-dependent and activity-independent processes. By engineering a mutated fetal AChR subunit, we used homologous recombination to develop a mouse line that expresses AChR with massively reduced open probability during embryonic development. Through histological and immunochemical methods as well as electrophysiological techniques, we observed that endplate anatomy and distribution are severely aberrant and innervation patterns are completely disrupted. Nonetheless, in the absence of activity AChRs form postsynaptic specializations attracting motor axons and permitting generation of multiple nerve/muscle contacts on individual fibers. This process is not restricted to a specialized central zone of the diaphragm and proceeds throughout embryonic development. Phenotypes can be attributed to separate activity-dependent and -independent pathways. The correct patterning of synaptic connections, prevention of multiple contacts and control of nerve growth require AChR-mediated activity. In contrast, myotube survival and acetylcholine-mediated dispersal of AChRs are maintained even in the absence of AChR-mediated activity. Because mouse models in which acetylcholine is entirely absent do not display similar effects, we conclude that acetylcholine binding to the AChR initiates activity-dependent and activity-independent pathways whereby the AChR modulates formation of the NMJ.

• Time lapse in vivo visualization of developmental stabilization of synaptic receptors at neuromuscular junctions.

  Authors: Pessah Yampolsky, Pier Giorgio Pacifici, Lukas Lomb, Günter Giese, Rüdiger Rudolf, Ira V Röder, Veit Witzemann

  The Journal of biological chemistry. 11/2010; 285(45):34589-96.

  The lifetime of nicotinic acetylcholine receptors (AChRs) in neuromuscular junctions (NMJs) is increased from <1 day to >1 week during early postnatal development. However, the exact timing of AChRThe lifetime of nicotinic acetylcholine receptors (AChRs) in neuromuscular junctions (NMJs) is increased from <1 day to >1 week during early postnatal development. However, the exact timing of AChR stabilization is not known, and its correlation to the concurrent embryonic to adult AChR channel conversion, NMJ remodeling, and neuromuscular diseases is unclear. Using a novel time lapse in vivo imaging technology we show that replacement of the entire receptor population of an individual NMJ occurs end plate-specifically within hours. This makes it possible to follow directly in live animals changing stabilities of end plate receptors. In three different, genetically modified mouse models we demonstrate that the metabolic half-life values of synaptic AChRs increase from a few hours to several days after postnatal day 6. Developmental stabilization is independent of receptor subtype and apparently regulated by an intrinsic muscle-specific maturation program. Myosin Va, an F-actin-dependent motor protein, is also accumulated synaptically during postnatal development and thus could mediate the stabilization of end plate AChR.

• Differential muscle-driven synaptic remodeling in the neuromuscular junction after denervation.

  Authors: Pessah Yampolsky, Pier Giorgio Pacifici, Veit Witzemann

  The European journal of neuroscience. 02/2010; 31(4):646-58.

  We used knock-in mice that express green fluorescent protein (GFP)-labeled embryonic-type acetylcholine receptors to investigate postsynaptic responses to denervation of fast-twitch and slow-twitchWe used knock-in mice that express green fluorescent protein (GFP)-labeled embryonic-type acetylcholine receptors to investigate postsynaptic responses to denervation of fast-twitch and slow-twitch muscle fibers, and to visualize the integration of newly synthesized GFP-labeled embryonic-type receptors into adult synapses. The embryonic-type receptors are transiently expressed and incorporated into the denervated endplates. They replaced synaptic adult-type receptors in a directed fashion, starting from the endplate's periphery and proceeding to its central regions. The progress of embryonic-type receptor expression with respect to transcriptional control is a transient, short-term activation mechanism. The less pronounced increase in the expression levels of the GFP-labeled receptors revealed a differential shift in the integration and degradation processes that constitute the dynamic equilibrium of the synaptic receptor pool. Therefore, we were able to model the changes in the total receptor load of the neuromuscular endplate following denervation as a function of the abundance of available receptors and the initial receptor load of the endplate.

• A mouse model for congenital myasthenic syndrome due to MuSK mutations reveals defects in structure and function of neuromuscular junctions.

  Authors: Frédéric Chevessier, Emmanuelle Girard, Jordi Molgó, Bartling Sönke, Jeanine Koenig, Daniel Hantaï, Veit Witzemann

  Human molecular genetics. 09/2008;

  In the muscle-specific tyrosine kinase receptor gene MUSK a heteroallelic missense and a null mutation were identified in a patient suffering from a congenital myasthenic syndrome. We generated oneIn the muscle-specific tyrosine kinase receptor gene MUSK a heteroallelic missense and a null mutation were identified in a patient suffering from a congenital myasthenic syndrome. We generated one mouse line carrying the homozygous missense mutation V789M in musk (musk(V789M/V789M) mice) and a second hemizygous line, resembling the patient genotype, with the V789M mutation on one allele and an allele lacking the kinase domain (musk(V789M/-)mice). We report here that musk(V789M/V789M) mice present no obvious abnormal phenotype regarding weight, muscle function, and viability. In contrast, adult musk(V789M/-) mice suffer from severe muscle weakness, exhibit shrinkage of pelvic and scapular regions, and hunchback. Musk(V789M/-) diaphragm develops less force upon direct or nerve-induced stimulation. A profound tetanic fade is observed following nerve-evoked muscle contraction and fatigue resistance is severely impaired upon a train of tetanic nerve stimulations. Electrophysiological measurements indicate that fatigable muscle weakness is due to impaired neurotransmission as observed in a patient suffering from a congenital myasthenic syndrome. The diaphragm of adult musk(V789M/-) mice exhibits pronounced changes in endplate architecture, distribution and innervation pattern. Thus, the missense mutation V789M in MuSK acts as a hypomorphic mutation and leads to insufficiency in MuSK function in musk(V789M/-) mutants. These mutant mice represent valuable models for elucidating the roles of MuSK for synapse formation, maturation, and maintenance as well as for studying the pathophysiology of a congenital myasthenic syndrome due to MuSK mutations.

• AChR channel conversion and AChR-adjusted neuronal survival during embryonic development.

  Authors: Pessah Yampolsky, Sven Gensler, Joseph McArdle, Veit Witzemann

  Molecular and cellular neurosciences. 04/2008; 37(3):634-45.

  We generated knock-in mice that express GFP-labeled embryonic-type acetylcholine receptors (AChR) to follow postsynaptic differentiation and innervation during embryonic development and to visualizeWe generated knock-in mice that express GFP-labeled embryonic-type acetylcholine receptors (AChR) to follow postsynaptic differentiation and innervation during embryonic development and to visualize the postnatally occurring channel conversion from embryonic- to adult-type AChR. The dynamics of AChRgamma/AChRepsilon conversion at the neuromuscular junction indicates that muscle-specific programs of receptor subunit gene transcription control AChR replacement. While conversion proceeds from peripheral to central regions for individual endplates, it does not occur simultaneously for all endplates. Although GFP-labeled receptors were expressed at reduced levels, the localization of postsynaptic sites and synapse formation was not noticeably altered. However, these alterations correlated with a striking reduction of motoneuron programmed cell death, transient increase of neurite growth and axon branching. This animal model refines the view on reciprocal neuromuscular signaling and suggests that motoneuron survival and axon branching are directly regulated by AChR function to enable optimal innervation and timing of neurally evoked muscle contraction.

• Development of the neuromuscular junction.

  Authors: Veit Witzemann

  Cell and tissue research. 12/2006; 326(2):263-71.

  The differentiation of the neuromuscular junction is a multistep process requiring coordinated interactions between nerve terminals and muscle. Although innervation is not needed for muscleThe differentiation of the neuromuscular junction is a multistep process requiring coordinated interactions between nerve terminals and muscle. Although innervation is not needed for muscle production, the formation of nerve-muscle contacts, intramuscular nerve branching, and neuronal survival require reciprocal signals from nerve and muscle to regulate the formation of synapses. Following the production of muscle fibers, clusters of acetylcholine receptors (AChRs) are concentrated in the central regions of the myofibers via a process termed "prepatterning". The postsynaptic protein MuSK is essential for this process activating possibly its own expression, in addition to the expression of AChR. AChR complexes (aggregated and stabilized by rapsyn) are thus prepatterned independently of neuronal signals in developing myofibers. ACh released by branching motor nerves causes AChR-induced postsynaptic potentials and positively regulates the localization and stabilization of developing synaptic contacts. These "active" contact sites may prevent AChRs clustering in non-contacted regions and counteract the establishment of additional contacts. ACh-induced signals also cause the dispersion of non-synaptic AChR clusters and possibly the removal of excess AChR. A further neuronal factor, agrin, stabilizes the accumulation of AChR at synaptic sites. Agrin released from the branching motor nerve may form a structural link specifically to the ACh-activated endplates, thereby enhancing MuSK kinase activity and AChR accumulation and preventing dispersion of postsynaptic specializations. The successful stabilization of prepatterned AChR clusters by agrin and the generation of singly innervated myofibers appear to require AChR-mediated postsynaptic potentials indicating that the differentiation of the nerve terminal proceeds only after postsynaptic specializations have formed.

• Synapse disassembly and formation of new synapses in postnatal muscle upon conditional inactivation of MuSK.

  Authors: Boris A. Hesser, Oliver Henschel, Veit Witzemann

  Molecular and cellular neurosciences. 04/2006; 31(3):470-80.

  The muscle-specific-kinase MuSK is required for the formation of acetylcholine receptor clusters during embryonic development, but its physiological role in adult muscle is not known. We used theThe muscle-specific-kinase MuSK is required for the formation of acetylcholine receptor clusters during embryonic development, but its physiological role in adult muscle is not known. We used the loxP/Cre system in mice to conditionally inactivate MuSK whereby expression of Cre recombinase increases during postnatal development. The MuSK-inactivated mice develop myasthenic symptoms and die prematurely due to severe muscle weakness. The postnatal inactivation of MuSK causes loss of acetylcholine receptors and disassembly of the postsynaptic organization and innervating axons retract but start to grow and branch extensively. Due to the mosaic expression of Cre recombinase, MuSK is not globally inactivated and new synapses are formed aberrantly patterned across the diaphragm. Our findings demonstrate that MuSK kinase activity is required throughout postnatal development to hold up MuSK and AChR levels at endplates. Thus, MuSK and AChR together maintain the functional and structural integrity of the postsynaptic architecture and prevent axon growth.

• Acetylcholine receptor channel subtype directs the innervation pattern of skeletal muscle.

  Authors: Michael Koenen, Christoph Peter, Alfredo Villarroel, Veit Witzemann, Bert Sakmann

  EMBO reports. 07/2005; 6(6):570-6.

  Acetylcholine receptors (AChRs) mediate synaptic transmission at the neuromuscular junction, and structural and functional analysis has assigned distinct functions to the fetalAcetylcholine receptors (AChRs) mediate synaptic transmission at the neuromuscular junction, and structural and functional analysis has assigned distinct functions to the fetal (alpha2beta(gamma)delta) and adult types of AChR (alpha2beta(epsilon)delta). Mice lacking the epsilon-subunit gene die prematurely, showing that the adult type is essential for maintenance of neuromuscular synapses in adult muscle. It has been suggested that the fetally and neonatally expressed AChRs are crucial for muscle differentiation and for the formation of the neuromuscular synapses. Here, we show that substitution of the fetal-type AChR with an adult-type AChR preserves myoblast fusion, muscle and end-plate differentiation, whereas it substantially alters the innervation pattern of muscle by the motor nerve. Mutant mice form functional neuromuscular synapses outside the central, narrow end-plate band region in the diaphragm, with synapses scattered over a wider muscle territory. We suggest that one function of the fetal type of AChR is to ensure an orderly innervation pattern of skeletal muscle.

• Mutation of single murine acetylcholine receptor subunits reveals differential contribution of P121 to acetylcholine binding and channel opening.

  Authors: Christoph Peter, Alon Korngreen, Veit Witzemann

  Pflügers Archiv : European journal of physiology. 07/2005; 450(3):178-84.

  The nicotinic acetylcholine receptor (AChR) is a heteropentameric, ligand-gated ion channel at the neuromuscular junction, where it is responsible for signal transduction between the motorneuron andThe nicotinic acetylcholine receptor (AChR) is a heteropentameric, ligand-gated ion channel at the neuromuscular junction, where it is responsible for signal transduction between the motorneuron and the muscle. Point mutations in the subunits of the receptor change the channel's electrophysiological properties and underlie inherited forms of muscle weakness, the congenital myasthenic syndromes. One point mutation (P121L) has been identified in the epsilon-subunit of patients suffering from the fast-channel congenital myasthenic syndrome, which is evoked by reduced AChR openings. We introduced the P121L mutation into all murine AChR subunits and performed electrophysiological studies in Xenopus laevis oocytes. The P121L mutation in the epsilon-subunit of the adult mouse AChR affected ligand binding and channel gating in a manner similar to that described for human AChR. At equivalent positions in the alpha- and beta-subunits, the mutation caused only minor electrophysiological changes. Mutation of the delta-subunit had similar, but less pronounced functional consequences compared to epsilonP121L, reflecting the asymmetry of the acetylcholine binding sites and the dominant effect of the alpha-epsilon site on channel opening.

• High-efficiency transfection of individual neurons using modified electrophysiology techniques.

  Authors: Jan Rathenberg, Thomas Nevian, Veit Witzemann

  Journal of neuroscience methods. 07/2003; 126(1):91-8.

  Transfection of cells by electroporation is a widely used and efficient method. Recently, it has been shown that single neurons in brain slice cultures can be transfected using micropipettes loadedTransfection of cells by electroporation is a widely used and efficient method. Recently, it has been shown that single neurons in brain slice cultures can be transfected using micropipettes loaded with plasmid DNA expression constructs. However, the transfection efficiencies were very low. Routine employment of single-cell electroporation (SCE) for transfection of neurons requires high and reliable efficiency together with good cell survival. Here, we describe the modification of electrophysiology techniques for SCE leading to very simple and efficient (up to 80%) transfection of neurons in organotypic rat hippocampus and mouse cortex slice cultures. Electroporation-mediated transfection was visualized in real-time by two-photon microscopy at the cellular level using fluorescently labeled oligonucleotides and plasmid DNA. Small oligonucleotides enter the cell immediately during pulse application while large plasmids remain localized for more than 10 min at the cell membrane before they enter the cell by an, as yet, unknown process. SCE does not affect the electrophysiology of transgene-expressing cells. Expression of several neuronal green fluorescent protein-tagged proteins demonstrates that the method can be employed to analyze subcellular trafficking and targeting in single living neurons.

• Modification of choline acetyltransferase by integration of green fluorescent protein does not affect enzyme activity and subcellular distribution.

  Authors: Jan Rathenberg, Annette Gärtner, Michael Koenen, Veit Witzemann

  Cell and tissue research. 05/2002; 308(1):1-6.

  Choline acetyltransferase (ChAT) is widely used as a marker enzyme to identify cholinergic neurons in the central and peripheral nervous system and to study developmental changes. In order toCholine acetyltransferase (ChAT) is widely used as a marker enzyme to identify cholinergic neurons in the central and peripheral nervous system and to study developmental changes. In order to visualize expression of ChAT directly we have generated a ChAT-green fluorescent protein (GFP) fusion construct. Upon transfection of COS-1 cells and cultured rat hippocampal neurons, transgenic enzymatically active ChAT-GFP is expressed and shows intrinsic fluorescence. In COS-1 cells the ChAT-GFP construct revealed a subcellular distribution indistinguishable from wild-type ChAT. In primary neurons the fluorescence was present in the soma and neuritic processes. Hence, this construct will be useful for analyzing the expression and subcellular distribution of ChAT-GFP in cell and tissue culture.

• Differential regulation of MyoD and myogenin mRNA levels by nerve induced muscle activity

  Authors: Veit Witzemann, Bert Sakmann

  FEBS Letters.

  The levels of mRNAs coding for the myogenic factors MyoD and myogenin were measured during synapse formulation in developing muscle and in adult muscle, after denervation and reinnervation and afterThe levels of mRNAs coding for the myogenic factors MyoD and myogenin were measured during synapse formulation in developing muscle and in adult muscle, after denervation and reinnervation and after muscle paralysis induced by blocking of neuromuscular transmission by neurotoxins known to alter the density and localization of synaptic proteins such as the acetylcholine receptor (AChR). The mRNA levels of both factors depend on usage of the neuromuscular synapses, but they change to different extents. Myogenin mRNA levels decrease drastically with innervation and increase strongly following blocking of transmission whereas the level of MyoD mRNA showed only a small decrease in response to innervation, denervation of muscle paralysis by neurotoxins. Neither mRNA showed a synapse-related cellular distribution. The results suggest that nerve-induced electrical muscle activity determines the cellular ratio of MyoD and myogenin mRNAs in adult muscle.

• Argiotoxin636 inhibits NMDA-activated ion channels expressed in Xenopus oocytes

  Authors: Andreas Draguhn, Werner Jahn, Veit Witzemann

  Neuroscience Letters.

  Argiotoxin636, a component of the spider venom of argiope species, was chemically synthesized together with a number of derivatives in order to analyse their blocking activity on mammalian glutamateArgiotoxin636, a component of the spider venom of argiope species, was chemically synthesized together with a number of derivatives in order to analyse their blocking activity on mammalian glutamate receptors. Xenopus laevis oocytes injected with rat brain mRNA served as assay system. The results showed that argiotoxin636 had a higher affinity for (NMDA) than for kainate receptors, blocking the corresponding ion channels in a voltage-dependent manner. Modifications of the polyamine tail or the terminal arginine residue strongly reduced the blocking potency. The iodinated monohydroxyl phenylderivatives, however, retained their NMDA-selective binding and could serve as non-competitive antagonists for radioligand binding assays aiding in the biochemical isolation of glutamate receptors.

• Differential regulation of muscle acetylcholine receptor γ-and ϵ-subunit mRNAs

  Authors: Veit Witzemann, Brigitte Barg, Yoshiki Nishikawa, Bert Sakmann, Shosaku Numa

  FEBS Letters.

  The contents of the mRNAs encoding the γ- and ϵ-subunits of the nicotinic acetylcholine receptor as well as the single-channel properties of the receptor have been assessed in innervated, denervatedThe contents of the mRNAs encoding the γ- and ϵ-subunits of the nicotinic acetylcholine receptor as well as the single-channel properties of the receptor have been assessed in innervated, denervated and reinnervated rat muscle. The changes in abundance of the γ- and ϵ-subunit mRNAs correlate with the changes in relative density of two classes of acetylcholine receptor channels. The results support the view that a switch in the relative abundance of the γ- and ϵ-subunit mRNAs is a major mechanism in regulating the properties of acetylcholine receptor channels in muscle.

• AChR channel conversion and AChR-adjusted neuronal survival during embryonic development

  Authors: Pessah Yampolsky, Sven Gensler, Joseph McArdle, Veit Witzemann

  Molecular and Cellular Neuroscience.

  We generated knock-in mice that express GFP-labeled embryonic-type acetylcholine receptors (AChR) to follow postsynaptic differentiation and innervation during embryonic development and to visualizeWe generated knock-in mice that express GFP-labeled embryonic-type acetylcholine receptors (AChR) to follow postsynaptic differentiation and innervation during embryonic development and to visualize the postnatally occurring channel conversion from embryonic- to adult-type AChR. The dynamics of AChRγ/AChRε conversion at the neuromuscular junction indicates that muscle-specific programs of receptor subunit gene transcription control AChR replacement. While conversion proceeds from peripheral to central regions for individual endplates, it does not occur simultaneously for all endplates. Although GFP-labeled receptors were expressed at reduced levels, the localization of postsynaptic sites and synapse formation was not noticeably altered. However, these alterations correlated with a striking reduction of motoneuron programmed cell death, transient increase of neurite growth and axon branching. This animal model refines the view on reciprocal neuromuscular signaling and suggests that motoneuron survival and axon branching are directly regulated by AChR function to enable optimal innervation and timing of neurally evoked muscle contraction.

• Subunit-specific block of cloned NMDA receptors by argiotoxin636

  Authors: Martin Raditsch, J.Peter Ruppersberg, Thomas Kuner, Willy Günther, Ralf Schoepfer, Peter H. Seeburg, Werner Jahn, Veit Witzemann

  FEBS Letters.

  Cloned NMDA receptor channels of the NR1-NR2A, NR1-NR2B and NR1-NR2C type show differences in argiotoxin636 block. Mutations of an asparagine residue located at a homologous position in the TM2Cloned NMDA receptor channels of the NR1-NR2A, NR1-NR2B and NR1-NR2C type show differences in argiotoxin636 block. Mutations of an asparagine residue located at a homologous position in the TM2 region of all NMDA receptor subunits, which corresponds to the Q/R site of the AMPA receptors, alters the argiotoxin636-induced block. The results suggest that the toxin interacts at this amino acid position with the putative pore forming TM2 region of the NMDA receptor subunits. Sequence differences in the TM2 segment of NR2A and NR2C subunits are not responsible for the subtype-specific sensitivity to argiotoxin636 as revealed by site-directed mutagenesis.

• Electrical Activity and Postsynapse Formation in Adult Muscle: γ-AChRs Are Not Required

  Authors: Said Hashemolhosseini, Chris Moore, Lukas Landmann, Andreas Sander, Holger Schwarz, Veit Witzemann, Bert Sakmann, Hans Rudolf Brenner

  Molecular and Cellular Neuroscience.

  Skeletal muscle fibers will not accept hyperinnervation by foreign motor axons unless they are paralyzed, suggesting that paralysis makes them receptive to innervation, e.g., by upregulatingSkeletal muscle fibers will not accept hyperinnervation by foreign motor axons unless they are paralyzed, suggesting that paralysis makes them receptive to innervation, e.g., by upregulating extrasynaptic expression of γ-AChRs and/or of the agrin receptor MuSK. To examine the involvement of these parameters in paralysis-mediated synapse induction, ectopic expression of agrin, a factor from motor neurons controlling neuromuscular synapse formation, was made dependent on the administration of doxycycline in innervated adult muscle fibers. In response to doxycycline-induced agrin secretion, adult fibers did form ectopic postsynaptic specializations, even when they were electrically active, lacked fetal AChRs, and expressed normal low levels of MuSK. These data demonstrate that paralysis and changes associated with it are not required for agrin-induced postsynapse formation. They suggest that paralyzed muscle induces synapse formation via the release of factors that make motor neurites contact muscle fibers and secrete agrin.

• Acetylcholine receptors: Their role in synapse formation and muscle function

  Authors: Veit Witzemann, Holger Schwarz, Michael Koenen

  Neuroforum, v.2, 181-188 (2000).

• Imaging of cell membrane proteins with a scanning tunneling microscope

  Authors: Heinrich Hörber, F. M. Schuler, Veit Witzemann, Klaus H. Schröter, Holger Müller, J. Peter Ruppersberg

  J. Vac. Sci. & Technol., v.2, 1214-1218 (1991).