Biomedical subjects

Eckart D Gundelfinger

Publications and source records attributed to Eckart D Gundelfinger.

At least 19 recordsLinked to original sources

PubMed2006

The immunolocalization of the synaptic glycoprotein neuroplastin differs substantially between the human and the rodent brain.

Neuroplastin is a cell adhesion molecule of the immunoglobulin superfamily that exists in two splice isoforms, np65/np55, and that was reported to play a prominent role in synaptic plasticity processes. The splice isoform np65 associates with synapses in an activity-dependent manner and has been shown to play a role for the induction of hippocampal long-term potentiation in rodents. We have therefore analyzed the distribution of neuroplastins in human brain. Neuroplastin is present in many neuronal cell types of the forebrain and cerebellum and immunoreactive label covers the cell soma, neurites and also puncta in the neuropil were visible. Interestingly, we found some remarkable species differences in the expression patterns of neuroplastins between the human and the rodent brain. In human brain np65 is prominently present in cerebellum while np55 is the predominant isoform in mouse and rat cerebellum. Moreover, the parasagittal stripe-type of staining seen with np55 in mouse cerebellum is not found in human brain. In addition we found no segregation of np65 immunolabel in hippocampal subregions like it was reported previously for the rat. These results might indicate different cellular functions of the molecule in different species.

Animals↗

PubMed2006

Neuronal Ca2+ signaling via caldendrin and calneurons.

The calcium sensor protein caldendrin is abundantly expressed in neurons and is thought to play an important role in different aspects of synapto-dendritic Ca2+ signaling. Caldendrin is highly abundant in the postsynaptic density of a subset of excitatory synapses in brain and its distinct localization raises several decisive questions about its function. Previous work suggests that caldendrin is tightly associated with Ca2+ - and Ca2+ release channels and might be involved in different aspects of the organization of the postsynaptic scaffold as well as with synapse-to-nucleus communication. In this report we introduce two new EF-hand calcium sensor proteins termed calneurons that apart from calmodulin represent the closest homologues of caldendrin in brain. Calneurons have a different EF-hand organization than other calcium sensor proteins, are prominently expressed in neurons and will presumably bind Ca2+ with higher affinity than caldendrin. Despite some significant structural differences it is conceivable that they are involved in similar Ca2+ regulated processes like caldendrin and neuronal calcium sensor proteins.

Amino Acid Sequence↗

PubMed2006

Molecular organization of the presynaptic active zone.

The exocytosis of neurotransmitter-filled synaptic vesicles is under tight temporal and spatial control in presynaptic nerve terminals. The fusion of synaptic vesicles is restricted to a specialized area of the presynaptic plasma membrane: the active zone. The protein network that constitutes the cytomatrix at the active zone (CAZ) is involved in the organization of docking and priming of synaptic vesicles and in mediating use-dependent changes in release during short-term and long-term synaptic plasticity. To date, five protein families whose members are highly enriched at active zones (Munc13s, RIMs, ELKS proteins, Piccolo and Bassoon, and the liprins-alpha), have been characterized. These multidomain proteins are instrumental for the diverse functions performed by the presynaptic active zone.

Animals↗

PubMed2006

A role for zinc in postsynaptic density asSAMbly and plasticity?

Chemical synapses are asymmetric cell junctions that mediate communication between neurons. Multidomain scaffolding proteins of the Shank family act as major organizing elements of the "postsynaptic density"--that is, the cytoskeletal protein matrix associated with the postsynaptic membrane. A recent study has shown that the C-terminal sterile alpha-motif or "SAM domain" of Shank3 (also known as ProSAP2) can form two-dimensional sheets of helical fibers. Assembly and packaging of these fibers are markedly enhanced by the presence of Zn2+ ions. Zn2+ can be released together with glutamate from synaptic vesicles and can enter the postsynaptic cell through specific ionotropic receptors. Based on these observations, we propose a new model of synaptic plasticity in which Zn2+ influx directly and instantly modulates the structure and function of the postsynaptic density.

Adaptor Proteins, Signal Transducing↗

PubMed2006

ProSAP-interacting protein 1 (ProSAPiP1), a novel protein of the postsynaptic density that links the spine-associated Rap-Gap (SPAR) to the scaffolding protein ProSAP2/Shank3.

ProSAPs/Shanks are a family of proteins that have a major scaffolding function for components of the postsynaptic density (PSD) of excitatory brain synapses. Members of the family harbor a variety of domains for protein-protein interactions, one of which is a unique PDZ domain that differs significantly from those of other proteins. We have identified a novel binding partner for this PDZ domain, termed ProSAPiP1, that is highly enriched in the PSD and shares significant sequence homology with the PSD protein PSD-Zip70. Both molecules code for a Fez1 domain that can be found in a total of four related proteins. ProSAPiP1 is widely expressed in rat brain and co-localizes with ProSAP2/Shank3 in excitatory spines and synapses. ProSAP2/Shank3 co-immunoprecipitates with ProSAPiP1 but not with PSD-Zip70. Both proteins, however, bind and recruit SPAR to synapses with a central coiled-coil region that harbors a leucine zipper motif. This region is also responsible for homo- and heteromultimerization of ProSAPiP1 and PSD-Zip70. Thus, ProSAPiP1 and PSD-Zip70 are founders of a novel family of scaffolding proteins, the "Fezzins," which adds further complexity to the organization of the PSD protein network.

Adaptor Proteins, Signal Transducing↗

PubMed2006

Brevican-containing perineuronal nets of extracellular matrix in dissociated hippocampal primary cultures.

Perineuronal nets (PNN) are specialized extracellular matrix structures enwrapping CNS neurons, which are important regulators for neuronal and synaptic functions. Brevican, a chondroitin sulfate proteoglycan, is an integral component of PNN. Here, we have investigated the appearance of these structures in hippocampal primary cultures. The expression profile of brevican in mixed cultures resembles the in vivo pattern with a strong upregulation of all isoforms during the second and 3rd weeks in culture. Brevican is primarily synthesized by co-cultured glial fibrillary acidic protein (GFAP-)-positive astrocytes and co-assembles with its interaction partners in PNN-like structures on neuronal somata and neurites as identified by counterstaining with the PNN marker Vicia villosa lectin. Both excitatory and inhibitory synapses are embedded into PNN. Furthermore, axon initial segments are strongly covered by a dense brevican coat. Altogether, we show that mature primary cultures can form PNN, and that basic features of these extracellular matrix structures may be studied in vitro.

Animals↗

PubMed2006

Manganese-enhanced MRI reveals structural and functional changes in the cortex of Bassoon mutant mice.

Manganese-enhanced magnetic resonance imaging (ME-MRI) was used to analyze the brain architecture in mice lacking the functional presynaptic active zone protein Bassoon. Anatomical characterization revealed a significant increase in the total brain volume in Bassoon mutants as compared with wild-type mice, which is mainly caused by changes in cortex and hippocampus volume. The measured enlargement in cortical volume coincides with an altered Mn2+ distribution within cortical layers as visualized by T1-weighted magnetic resonance imaging. Two days after manganese application, the cortex of Bassoon mutant mice appeared more laminated in ME-MRI, with an enhanced accumulation of manganese in deep, central, and superficial cortical cell layers. Whereas morphologically the cortical lamination is not affected by the absence of a functional Bassoon, an altered basal activation pattern was found in the cortex of the mutant mice both by metabolic labeling with [14C]-2-deoxyglucose and histochemical detection of the potassium analogue thallium uptake. Consequently, the results indicate that the absence of the functional presynaptic protein Bassoon causes disturbance in the formation of normal basal cortical activation patterns and thereby in the functional cortical architecture. Furthermore, this study shows that ME-MRI can become a valuable tool for a structural characterization of genetically modified mice.

Animals↗

PubMed2006

The dopaminergic midbrain participates in human episodic memory formation: evidence from genetic imaging.

Recent data from animal studies raise the possibility that dopaminergic neuromodulation promotes the encoding of novel stimuli. We investigated a possible role for the dopaminergic midbrain in human episodic memory by measuring how polymorphisms in dopamine clearance pathways affect encoding-related brain activity (functional magnetic resonance imaging) in an episodic memory task. In 51 young, healthy adults, successful episodic encoding was associated with activation of the substantia nigra. This midbrain activation was modulated by a functional variable number of tandem repeat (VNTR) polymorphism in the dopamine transporter (DAT1) gene. Despite no differences in memory performance between genotype groups, carriers of the (low expressing) 9-repeat allele of the DAT1 VNTR showed relatively higher midbrain activation when compared with subjects homozygous for the 10-repeat allele, who express DAT1 at higher levels. The catechol-O-methyl transferase (COMT) Val108/158Met polymorphism, which is known to modulate enzyme activity, affected encoding-related activity in the right prefrontal cortex (PFC) and in occipital brain regions but not in the midbrain. Moreover, subjects homozygous for the (low activity) Met allele showed stronger functional coupling between the PFC and the hippocampus during encoding. Our finding that genetic variations in the dopamine clearance pathways affect encoding-related activation patterns in midbrain and PFC provides strong support for a role of dopaminergic neuromodulation in human episodic memory formation. It also supports the hypothesis of anatomically and functionally distinct roles for DAT1 and COMT in dopamine metabolism, with DAT1 modulating rapid, phasic midbrain activity and COMT being particularly involved in prefrontal dopamine clearance.

Adolescent↗

PubMed2006

An architectural framework that may lie at the core of the postsynaptic density.

The postsynaptic density (PSD) is a complex assembly of proteins associated with the postsynaptic membrane that organizes neurotransmitter receptors, signaling pathways, and regulatory elements within a cytoskeletal matrix. Here we show that the sterile alpha motif domain of rat Shank3/ProSAP2, a master scaffolding protein located deep within the PSD, can form large sheets composed of helical fibers stacked side by side. Zn2+, which is found in high concentrations in the PSD, binds tightly to Shank3 and may regulate assembly. Sheets of the Shank protein could form a platform for the construction of the PSD complex.

Adaptor Proteins, Signal Transducing↗

PubMed2006

Molecular organization and assembly of the presynaptic active zone of neurotransmitter release.

At chemical synapses, neurotransmitter is released at a restricted region of the presynaptic plasma membrane, called the active zone. At the active zone, a matrix of proteins is assembled, which is termed the presynaptic grid or cytomatrix at the active zone (CAZ). Components of the CAZ are thought to localize and organize the synaptic vesicle cycle, a series of membrane trafficking events underlying regulated neurotransmitter exocytosis. This review is focused on a set of specific proteins involved in the structural and functional organization of the CAZ. These include the multi-domain Rab3-effector proteins RIM1alpha and RIM2alpha; Bassoon and Piccolo, two multi-domain CAZ scaffolding proteins of enormous size; as well as members of the CAST/ERC family of CAZ-specific structural proteins. Studies on ribbon synapses of retinal photoreceptor cells have fostered understanding the molecular design of the CAZ. In addition, the analysis of the delivery pathways for Bassoon and Piccolo to presynaptic sites during development has produced new insights into assembly mechanisms of brain synapses during development. Based on these studies, the active zone transport vesicle hypothesis was formulated, which postulates that active zones, at least in part, are pre-assembled in neuronal cell bodies and transported as so-called Piccolo-Bassoon transport vesicles (PTVs) to sites of synaptogenesis. Several PTVs can fuse on demand with the presynaptic membrane to rapidly form an active zone.

Biological Transport↗

PubMed2006

Analysis of synaptic ultrastructure without fixative using high-pressure freezing and tomography.

Electron microscopy allows the analysis of synaptic ultrastructure and its modifications during learning or in pathological conditions. However, conventional electron microscopy uses aldehyde fixatives that alter the morphology of the synapse by changing osmolarity and collapsing its molecular components. We have used high-pressure freezing (HPF) to capture within a few milliseconds structural features without aldehyde fixative, and thus to provide a snapshot of living synapses. CA1 hippocampal area slices from P21 rats were frozen at -173 degrees C under high pressure to reduce crystal formation, and synapses on dendritic spines were analysed after cryosubstitution and embedding. Synaptic terminals were larger than after aldehyde fixation, and synaptic vesicles in these terminals were less densely packed. Small filaments linked the vesicles in subgroups. The postsynaptic densities (PSDs) exhibited filamentous projections extending into the spine cytoplasm. Tomographic analysis showed that these projections were connected with the spine cytoskeletal meshwork. Using immunocytochemistry, we found as expected GluR1 at the synaptic cleft and CaMKII in the PSD. Actin immunoreactivity (IR) labelled the cytoskeletal meshwork beneath the filamentous projections, but was very scarce within the PSD itself. ProSAP2/Shank3, cortactin and Ena/VASP-IRs were concentrated on the cytoplasmic face of the PSD, at the level of the PSD projections. Synaptic ultrastructure after HPF was different from that observed after aldehyde fixative. The boutons were larger, and filamentous components were preserved. Particularly, filamentous projections were observed linking the PSD to the actin cytoskeleton. Thus, synaptic ultrastructure can be analysed under more realistic conditions following HPF.

Aldehydes↗

PubMed2006

The cell adhesion molecule neuroplastin-65 inhibits hippocampal long-term potentiation via a mitogen-activated protein kinase p38-dependent reduction in surface expression of GluR1-containing glutamate receptors.

Neuroplastin-65 is a brain-specific, synapse-enriched member of the immunoglobulin (Ig) superfamily of cell adhesion molecules. Previous studies highlighted the importance of neuroplastin-65 for long-term potentiation (LTP), but the mechanism was unclear. Here, we show how neuroplastin-65 activation of mitogen-activated protein kinase p38 (p38MAPK) modified synapse strength by altering surface glutamate receptor expression. Organotypic hippocampal slice cultures treated with the complete extracellular fragment of neuroplastin-65 (FcIg1-3) sustained an increase in the phosphorylation of p38MAPK and an inability to induce LTP at hippocampal synapses. The LTP block was reversed by application of the p38MAPK inhibitor SB202190, suggesting that p38MAPK activation occurred downstream of neuroplastin-65 binding and upstream of the loss of LTP. Further investigation revealed that the mechanism underlying neuroplastin-65-dependent prevention of LTP was a p38MAPK-dependent acceleration of the loss of surface-exposed glutamate receptor subunits that was reversed by pretreatment with the p38MAPK inhibitor SB202190. Our results indicate that neuroplastin-65 binding and associated stimulation of p38MAPK activity are upstream of a mechanism to control surface glutamate receptor expression and thereby influence plasticity at excitatory hippocampal synapses.

Animals↗

PubMed2006

Local sharing as a predominant determinant of synaptic matrix molecular dynamics.

Recent studies suggest that central nervous system synapses can persist for weeks, months, perhaps lifetimes, yet little is known as to how synapses maintain their structural and functional characteristics for so long. As a step toward a better understanding of synaptic maintenance we examined the loss, redistribution, reincorporation, and replenishment dynamics of Synapsin I and ProSAP2/Shank3, prominent presynaptic and postsynaptic matrix molecules, respectively. Fluorescence recovery after photobleaching and photoactivation experiments revealed that both molecules are continuously lost from, redistributed among, and reincorporated into synaptic structures at time-scales of minutes to hours. Exchange rates were not affected by inhibiting protein synthesis or proteasome-mediated protein degradation, were accelerated by stimulation, and greatly exceeded rates of replenishment from somatic sources. These findings indicate that the dynamics of key synaptic matrix molecules may be dominated by local protein exchange and redistribution, whereas protein synthesis and degradation serve to maintain and regulate the sizes of local, shared pools of these proteins.

Animals↗

PubMed2005

Assembly of active zone precursor vesicles: obligatory trafficking of presynaptic cytomatrix proteins Bassoon and Piccolo via a trans-Golgi compartment.

Neurotransmitter release from presynaptic nerve terminals is restricted to specialized areas of the plasma membrane, so-called active zones. Active zones are characterized by a network of cytoplasmic scaffolding proteins involved in active zone generation and synaptic transmission. To analyze the modes of biogenesis of this cytomatrix, we asked how Bassoon and Piccolo, two prototypic active zone cytomatrix molecules, are delivered to nascent synapses. Although these proteins may be transported via vesicles, little is known about the importance of a vesicular pathway and about molecular determinants of cytomatrix molecule trafficking. We found that Bassoon and Piccolo co-localize with markers of the trans-Golgi network in cultured neurons. Impairing vesicle exit from the Golgi complex, either using brefeldin A, recombinant proteins, or a low temperature block, prevented transport of Bassoon out of the soma. Deleting a newly identified Golgi-binding region of Bassoon impaired subcellular targeting of recombinant Bassoon. Overexpressing this region to specifically block Golgi binding of the endogenous protein reduced the concentration of Bassoon at synapses. These results suggest that, during the period of bulk synaptogenesis, a primordial cytomatrix assembles in a trans-Golgi compartment. They further indicate that transport via Golgi-derived vesicles is essential for delivery of cytomatrix proteins to the synapse. Paradigmatically this establishes Golgi transit as an obligatory step for subcellular trafficking of distinct cytoplasmic scaffolding proteins.

Animals↗

PubMed2005

Ca2+-independent binding and cellular expression profiles question a significant role of calmyrin in transduction of Ca2+-signals to Alzheimer's disease-related presenilin 2 in forebrain.

The interaction between the EF-hand Ca(2+)-binding protein calmyrin and presenilin 2 (PS2) has been suggested to play a role in Alzheimer's disease (AD). We now report that calmyrin binds specifically endogenous PS2 and not PS1. However, binding appears to be Ca(2+)-independent and calmyrin does not exhibit a Ca(2+)-dependent translocation to intracellular membranes as demonstrated in a Ca(2+)-myristoyl switch assay. Moreover, calmyrin is only present at very low levels in brain areas associated with the onset of AD. In rat, forebrain calmyrin is localized only in a subset of principal neurons, similarly as in human forebrain. Finally, subcellular fractionation demonstrates only a limited overlap of calmyrin and PS2 at neuronal membranes. We therefore conclude that calmyrin will not contribute significantly as a Ca(2+)-sensor that transduces Ca(2+)-signaling events to PS2 in forebrain.

Alzheimer Disease↗

PubMed2005

Semaphorin 4B interacts with the post-synaptic density protein PSD-95/SAP90 and is recruited to synapses through a C-terminal PDZ-binding motif.

The semaphorins are a large family of proteins that act as guidance signals for axons and dendrites. The class 4 semaphorins are integral membrane proteins that are widely expressed throughout the nervous system. Here, we show that a subclass of these semaphorins is characterized by a PDZ-binding motif at their carboxy-terminus. This sequence mediates the interaction with the post-synaptic density protein PSD-95/SAP90. Co-expression of Sema4B with PSD-95 in COS 7 cells results in the clustering of Sema4B. Sema4B co-localizes with PSD-95 at synaptic contacts between cultured hippocampal neurons. This synaptic localization depends on the presence of the PDZ-binding motif.

Amino Acid Motifs↗

PubMed2005

Hair cell synaptic ribbons are essential for synchronous auditory signalling.

Hearing relies on faithful synaptic transmission at the ribbon synapse of cochlear inner hair cells (IHCs). At present, the function of presynaptic ribbons at these synapses is still largely unknown. Here we show that anchoring of IHC ribbons is impaired in mouse mutants for the presynaptic scaffolding protein Bassoon. The lack of active-zone-anchored synaptic ribbons reduced the presynaptic readily releasable vesicle pool, and impaired synchronous auditory signalling as revealed by recordings of exocytic IHC capacitance changes and sound-evoked activation of spiral ganglion neurons. Both exocytosis of the hair cell releasable vesicle pool and the number of synchronously activated spiral ganglion neurons co-varied with the number of anchored ribbons during development. Interestingly, ribbon-deficient IHCs were still capable of sustained exocytosis with normal Ca2+-dependence. Endocytic membrane retrieval was intact, but an accumulation of tubular and cisternal membrane profiles was observed in ribbon-deficient IHCs. We conclude that ribbon-dependent synchronous release of multiple vesicles at the hair cell afferent synapse is essential for normal hearing.

Action Potentials↗

PubMed2005

Molecular dissection of the photoreceptor ribbon synapse: physical interaction of Bassoon and RIBEYE is essential for the assembly of the ribbon complex.

The ribbon complex of retinal photoreceptor synapses represents a specialization of the cytomatrix at the active zone (CAZ) present at conventional synapses. In mice deficient for the CAZ protein Bassoon, ribbons are not anchored to the presynaptic membrane but float freely in the cytoplasm. Exploiting this phenotype, we dissected the molecular structure of the photoreceptor ribbon complex. Identifiable CAZ proteins segregate into two compartments at the ribbon: a ribbon-associated compartment including Piccolo, RIBEYE, CtBP1/BARS, RIM1, and the motor protein KIF3A, and an active zone compartment including RIM2, Munc13-1, a Ca2+ channel alpha1 subunit, and ERC2/CAST1. A direct interaction between the ribbon-specific protein RIBEYE and Bassoon seems to link the two compartments and is responsible for the physical integrity of the photoreceptor ribbon complex. Finally, we found the RIBEYE homologue CtBP1 at ribbon and conventional synapses, suggesting a novel role for the CtBP/BARS family in the molecular assembly and function of central nervous system synapses.

Alcohol Oxidoreductases↗