Volker Eulenburg

Inactivation of the glycine transporter 1 gene discloses vital role of glial glycine uptake in glycinergic inhibition.

The glycine transporter subtype 1 (GlyT1) is widely expressed in astroglial cells throughout the mammalian central nervous system and has been implicated in the regulation of N-methyl-D-aspartate (NMDA) receptor activity. Newborn mice deficient in GlyT1 are anatomically normal but show severe motor and respiratory deficits and die during the first postnatal day. In brainstem slices from GlyT1-deficient mice, in vitro respiratory activity is strikingly reduced but normalized by the glycine receptor (GlyR) antagonist strychnine. Conversely, glycine or the GlyT1 inhibitor sarcosine suppress respiratory activity in slices from wild-type mice. Thus, during early postnatal life, GlyT1 is essential for regulating glycine concentrations at inhibitory GlyRs, and GlyT1 deletion generates symptoms found in human glycine encephalopathy.

Deletion of the Mouse Glycine Transporter 2 Results in a Hyperekplexia Phenotype and Postnatal Lethality

The glycine transporter subtype 2 (GlyT2) is localized in the axon terminals of glycinergic neurons. Mice deficient in GlyT2 are normal at birth but during the second postnatal week develop a lethal neuromotor deficiency that resembles severe forms of human hyperekplexia (hereditary startle disease) and is characterized by spasticity, tremor, and an inability to right. Histological and immunological analyses failed to reveal anatomical or biochemical abnormalities, but the amplitudes of glycinergic miniature inhibitory currents (mIPSCs) were strikingly reduced in hypoglossal motoneurons and dissociated spinal neurons from GlyT2-deficient mice. Thus, postnatal GlyT2 function is crucial for efficient transmitter loading of synaptic vesicles in glycinergic nerve terminals, and the GlyT2 gene constitutes a candidate disease gene in human hyperekplexia patients.

Impaired GABAergic transmission and altered hippocampal synaptic plasticity in collybistin-deficient mice

Collybistin (Cb) is a brain‐specific guanine nucleotide exchange factor that has been implicated in plasma membrane targeting of the postsynaptic scaffolding protein gephyrin found at glycinergic and GABAergic synapses. Here we show that Cb‐deficient mice display a region‐specific loss of postsynaptic gephyrin and GABAA receptor clusters in the hippocampus and the basolateral amygdala. Cb deficiency is accompanied by significant changes in hippocampal synaptic plasticity, due to reduced dendritic GABAergic inhibition. Long‐term potentiation is enhanced, and long‐term depression reduced, in Cb‐deficient hippocampal slices. Consistent with the anatomical and electrophysiological findings, the animals show increased levels of anxiety and impaired spatial learning. Together, our data indicate that Cb is essential for gephyrin‐dependent clustering of a specific set of GABAA receptors, but not required for glycine receptor postsynaptic localization.

Neurotransmitter transporters expressed in glial cells as regulators of synapse function.

Synaptic neurotransmission at high temporal and spatial resolutions requires efficient removal and/or inactivation of presynaptically released transmitter to prevent spatial spreading of transmitter by diffusion and allow for fast termination of the postsynaptic response. This action must be carefully regulated to result in the fine tuning of inhibitory and excitatory neurotransmission, necessary for the proper processing of information in the central nervous system. At many synapses, high-affinity neurotransmitter transporters are responsible for transmitter deactivation by removing it from the synaptic cleft. The most prevailing neurotransmitters, glutamate, which mediates excitatory neurotransmission, as well as GABA and glycine, which act as inhibitory neurotransmitters, use these uptake systems. Neurotransmitter transporters have been found in both neuronal and glial cells, thus suggesting high cooperativity between these cell types in the control of extracellular transmitter concentrations. The generation and analysis of animals carrying targeted disruptions of transporter genes together with the use of selective inhibitors have allowed examining the contribution of individual transporter subtypes to synaptic transmission. This revealed the predominant role of glial expressed transporters in maintaining low extrasynaptic neurotransmitter levels. Additionally, transport activity has been shown to be actively regulated on both transcriptional and post-translational levels, which has important implications for synapse function under physiological and pathophysiological conditions. The analysis of these mechanisms will enhance not only our understanding of synapse function but will reveal new therapeutic strategies for the treatment of human neurological diseases.

Glycine transporters : essential regulators of synaptic transmission

Glycine is a major inhibitory neurotransmitter in the mammalian CNS (central nervous system). Glycinergic neurotransmission is terminated by the uptake of glycine into glycinergic nerve terminals and neighbouring glial cells. This uptake process is mediated by specific Na(+)/Cl(-)-dependent GlyTs (glycine transporters), GlyT1 and GlyT2. GlyT1, in addition, is thought to regulate the concentration of glycine at excitatory synapses containing NMDARs (N-methyl-D-aspartate receptors), which require glycine as a co-agonist. We have analysed the physiological roles and regulation of GlyT1 and GlyT2 by generating transporter-deficient mice and searching for interacting proteins. Our genetic results indicate that at glycinergic synapses, the glial transporter GlyT1 catalyses the removal of glycine from the synaptic cleft, whereas GlyT2 is required for the re-uptake of glycine into nerve terminals, thereby allowing for neurotransmitter reloading of synaptic vesicles. Both GlyT1 and GlyT2 are essential for CNS function, as revealed by the lethal phenotypes of the respective knockout mice. Mice expressing only a single GlyT1 allele are phenotypically normal but may have enhanced NMDAR function. GlyT2 is highly enriched at glycinergic nerve terminals, and Ca(2+)-triggered exocytosis and internalization are thought to regulate GlyT2 numbers in the pre-synaptic plasma membrane. We have identified different interacting proteins that may play a role in GlyT2 trafficking and/or pre-synaptic localization.

N-Cadherin Transsynaptically Regulates Short-Term Plasticity at Glutamatergic Synapses in Embryonic Stem Cell-Derived Neurons

The cell adhesion molecule N-cadherin has been proposed to regulate synapse formation in mammalian central neurons. This is based on its synaptic localization enabling alignment of presynaptic and postsynaptic specializations by an adhesion mechanism. However, a potential role of N-cadherin in regulating synaptic transmission has remained elusive. In this paper, a functional analysis of N-cadherin knock-out synapses was enabled by in vitro neuronal differentiation of mouse embryonic stem cells circumventing the early embryonic lethality of mice genetically null for N-cadherin. In our in vitro system, initial synapse formation was not altered in the absence of N-cadherin, which might be attributable to compensatory mechanisms. Here, we demonstrate that N-cadherin is required for regulating presynaptic function at glutamatergic synapses. An impairment in the availability of vesicles for exocytosis became apparent selectively during high activity. Short-term plasticity was strongly altered with synaptic depression enhanced in the absence of N-cadherin. Most intriguingly, facilitation was converted to depression under specific stimulation conditions. This indicates an important role of N-cadherin in the control of short-term plasticity. To analyze, whether N-cadherin regulates presynaptic function by a transsynaptic mechanism, we studied chimeric cultures consisting of wild-type neocortical neurons and ES cell-derived neurons. With N-cadherin absent only postsynaptically, we observed a similar increase in short-term synaptic depression as found in its complete absence. This indicates a retrograde control of short-term plasticity by N-cadherin. In summary, our results revealed an unexpected involvement of a synaptic adhesion molecule in the regulation of short-term plasticity at glutamatergic synapses.

The Adenomatous Polyposis Coli-protein (APC) interacts with the protein tyrosine phosphatase PTP-BL via an alternatively spliced PDZ domain

Mutations of the tumor suppressor protein APC (Adenomatous Polyposis Coli) are linked to familiar and sporadic human colon cancer. Here we describe a novel interaction between the APC protein and the protein tyrosine phosphatase PTP-BL carrying five PDZ protein–protein interaction domains. Exclusively, the second PDZ domain (PDZ2) of PTP-BL is binding to the extreme C-terminus of the APC protein, as determined by yeast two-hybrid studies. Using surface plasmon resonance analysis we established a dissociation constant (KD) of 8.1×10−9 M. We find that a naturally occurring splice insertion of five amino acids (PDZ2b) abolishes its binding affinity to the APC protein. The in vivo interaction between PTP-BL and the APC protein was shown by coprecipitation experiments in transfected COS cells. Furthermore, in cultured epithelial Madine Carnine Kidney cells the subcellular colocalization was demonstrated for the nucleus and also for the tips of cellular extensions. The interaction of the APC protein with a protein tyrosine phosphatase may indirectly modulate the steady state levels of tyrosine phosphorylations of associated proteins, such as β-catenin playing a major role in the regulation of cell division, migration and cell adhesion.

Glycine Transporter Dimers EVIDENCE FOR OCCURRENCE IN THE PLASMA MEMBRANE

Different Na+/Cl--dependent neurotransmitter transporters of the SLC6a family have been shown to form dimers or oligomers in both intracellular compartments and at the cell surface. In contrast, the glycine transporters (GlyTs) GlyT1 and -2 have been reported to exist as monomers in the plasma membrane based on hydrodynamic and native gel electrophoretic studies. Here, we used cysteine substitution and oxidative cross-linking to show that of GlyT1 and GlyT2 also form dimeric complexes within the plasma membrane. GlyT oligomerization at the cell surface was confirmed for both GlyT1 and GlyT2 by fluorescence resonance energy transfer microscopy. Endoglycosidase treatment and surface biotinylation further revealed that complex-glycosylated GlyTs form dimers located at the cell surface. Furthermore, substitution of tryptophan 469 of GlyT2 by an arginine generated a transporter deficient in dimerization that was retained intracellulary. Based on these results and GlyT structures modeled by using the crystal structure of the bacterial homolog LeuTAa, as a template, residues located within the extracellular loop 3 and at the beginning of transmembrane domain 6 are proposed to contribute to the dimerization interface of GlyTs.

Collybistin is required for both the formation and maintenance of GABAergic postsynapses in the hippocampus

Collybistin (Cb) is a brain-specific guanine nucleotide exchange factor, which interacts with the inhibitory receptor anchoring protein gephyrin. In the hippocampus of constitutively Cb-deficient adult mice, gephyrin and gephyrin-dependent GABA(A) receptors (GABA(A)Rs) are lost from postsynaptic sites. Here, we used a Cre-loxP system to inactivate the Cb gene in the forebrain at different developmental stages. Deletion of Cb during embryonic development prevented gephyrin clustering during synaptogenesis and caused an accumulation of gephyrin aggregates in the cell body of CA1 pyramidal neurons. Inactivation of the Cb gene during the third postnatal week resulted in a protracted loss of postsynaptic gephyrin clusters and the appearance of cytoplasmic gephyrin aggregates. These changes in gephyrin distribution were accompanied by a similar reduction in synaptically localized GABA(A)R gamma2-subunit immunoreactivity. Our data show that Cb is required for both the initial localization and maintenance of gephyrin and gephyrin-dependent GABA(A)Rs at inhibitory postsynaptic membrane specializations in the hippocampus.

PH-Domain-driven targeting of collybistin but not Cdc42 activation is required for synaptic gephyrin clustering

Collybistin (Cb) is a brain‐specific guanine nucleotide exchange factor (GEF) that is essential for the synaptic clustering of gephyrin and GABAA receptors in selected regions of the mammalian central nervous system. It has been previously proposed that Cb regulates gephyrin clustering by activating Cdc42, and thus acts as a signal transducer in a membrane activation process which labels postsynaptic membrane domains for inhibitory synapse formation. Here, we dissected the functional roles of the Dbl‐homology (DH) and pleckstrin homology (PH) domains of the constitutively active splice variant Cb II by substituting conserved amino acid residues that are required for GEF activity towards Cdc42 and phosphoinositide binding, respectively. A Cb II mutant lacking any detectable GEF activity towards Cdc42 was still fully active in inducing gephyrin scaffold formation, both in transfected NIH‐3T3 cells and in cultured hippocampal neurons. Furthermore, mice with a forebrain‐specific inactivation of the Cdc42 gene displayed normal densities of gephyrin and GABAA receptor clusters in the hippocampus. In contrast, substitution of Cb II PH‐domain residues essential for phosphoinositide binding abolished gephyrin recruitment to synaptic sites. Our results provide evidence that the formation of gephyrin scaffolds at inhibitory synapses requires an intact Cb II PH‐domain but is Cdc42‐independent.