Content
2008
Chemical Cross-linking Provides a Model of the y-Secretase Complex Subunit Architecture and Evidence for Close Proximity of the C-terminal Fragment of Presenilin with APH-1
12.12.2008
y-Secretase is an intramembrane cleaving aspartyl protease
complex intimately implicated in Alzheimer disease pathogenesis. The protease is composed of the catalytic subunit presenilin (PS1 or PS2), the substrate receptor nicastrin (NCT), and two additional subunits, APH-1 (APH-1a, as long and short splice forms (APH-1aL, APH-1aS), or APH-1b) and PEN-2. Apart
from the Alzheimer disease-associated b-amyloid precursor
protein, y-secretase has been shown to cleave a large number of other type I membrane proteins. Despite the progress in elucidating y-secretase function, basic questions concerning the precise organization of its subunits, their molecular interactions, and their exact stoichiometry in the complex are largely unresolved. Here we isolated endogenous human y-secretase from human embryonic kidney 293 cells and investigated the subunit architecture of the y-secretase complex formed by PS1, NCT, APH-1aL, and PEN-2 by chemical cross-linking. Using this approach, we provide evidence for the close neighborhood of the PS1 N- and C-terminal fragments (NTF and CTF, respectively), the PS1 NTF and PEN-2, the PS1 CTF and APH-1aL, and NCT and APH-1aL. We thus identify a previously unrecognized PS1 CTF/APH-1aL interaction, verify subunit interactions deduced previously from indirect approaches, and provide a model of the y-secretase complex subunit architecture. finally, we further show that, like the PS1 CTF, the PS2 CTF also interacts with APH-1aL, and we provide evidence that these interactions also occur with the other APH-1 variants, suggesting similar subunit architectures of all y-secretase complexes.
NR4A2 controls the differentiation of selective dopaminergic nuclei in the zebrafish brain
13.08.2008
MCN,
2008,
39 No.4,
592-604
published on 01.12.2008
Molecular and Cellular Neuroscience, online article
Molecular and Cellular Neuroscience, online article
The orphan nuclear receptor NR4A2/Nurr1 is mandatory for the terminal differentiation of mesencephalic dopamine neurons in mammals, but a similar role has remained elusive in the homologous area of the fish brain, the posterior tuberculum. Using loss- and gain-of-function experiments in zebrafish, we show that NR4A2 is indeed responsible for the expression of tyrosine hydroxylase (TH) in selective subpopulations of
dopamine cells in the posterior tuberculum, as well as in the pretectum, preoptic area and telencephalon. Cross sections of the neural tube reveal that cells expressing the proliferation marker PCNA, NR4A2 and TH are aligned along a mediolateral progression rather than overlapping populations, suggesting that NR4A2 does not simply regulate TH expression but also controls more general steps of progenitor commitment towards the fully differentiated DA neuronal state. Finally, in line with NR4A2+/− heterozygote mice, NR4A2 morphant fish are hyperactive. This behavioural phenotype is maintained throughout life, pointing to a developmental control of locomotor activity by NR4A2. Our results shed new light on NR4A2 function in the DA differentiation pathway, and stress the effect of DA dysregulation on the control of locomotor activity.
Identification of neural progenitor pools by E(Spl) factors in the embryonic and adult brain
20.11.2008
Brain Research Bulletin,
2008,
75,
266-273
published on 20.11.2008
Brain Research Bulletin, online article
Brain Research Bulletin, online article
The maintenance of progenitor cells is a crucial aspect of central nervous system development and maturation, and bHLH transcription factors of the E(Spl) subfamily are involved in this process in all vertebrates studied to date. In the zebrafish embryonic neural plate, a large number of E(Spl) genes (her genes) are at play.We review recent data on this point, and propose a model where distinct subsets of these genes define different progenitor subtypes. Analysis of her genes expression in the adult zebrafish brain suggests that part of the embryonic her cascade might also be
reused to define progenitors during adulthood. Further, available evidence on orthologous genes in the mouse (Hes genes) point to different modes of Hes regulation depending on cell location within the embryonic neural tube, perhaps associated with distinct progenitor types in this species as well. Out of these comparisons emerges a simple model of neural stem cell maintenance applicable from embryonic development until adulthood as well as across species. This working model suggests the directions for future experiments.
Ex vivo imaging of motor axon dynamics in murine triangularis sterni explants
17.11.2008
We provide a protocol that describes an explant system that allows the dynamics of motor axons to be imaged. This method is based on nerve–muscle explants prepared from the triangularis sterni
muscle of mice, a thin muscle that covers the inside of the thorax. These explants, which can be maintained alive for several hours, contain long stretches of peripheral motor axons including their
terminal arborizations and neuromuscular junctions. Explants can be prepared from transgenic mouse lines that express fluorescent proteins in neurons or glial cells, which enables direct visualization of their cellular and subcellular morphology by fluorescence microscopy. Time-lapse imaging then provides a convenient and reliable approach to follow the dynamic behavior of motor axons, their surrounding glial cells and their intracellular organelles with high temporal and spatial resolution. Triangularis sterni explants can be prepared in 15 min, imaged ex vivo for several hours and processed for immunohistochemistry in about 2 h.
Axonal Projections Originating From Raphe Serotonergic Neurons in the Developing and Adult Zebrafish, Danio rerio, Using Transgenics To Visualize Raphe-Specific pet1 Expression
10.11.2008
JCN,
2008,
512 No.2,
158-182
published on 10.11.2008
The Journal of Comparative Neurology, online article
The Journal of Comparative Neurology, online article
Serotonin is a major central nervous modulator of physiology
and behavior and plays fundamental roles during development
and plasticity of the vertebrate central nervous system
(CNS). Understanding the developmental control and
functions of serotonergic neurons is therefore an important
task. In all vertebrates, prominent serotonergic neurons are
found in the superior and inferior raphe nuclei in the hindbrain
innervating most CNS regions. In addition, all vertebrates
except for mammals harbor other serotonergic centers,
including several populations in the diencephalon. This, in combination with the intricate and wide distribution
of serotonergic fibers, makes it difficult to sort out serotonergic
innervation originating from the raphe from that of
other serotonergic cell populations. To resolve this issue,
we isolated the regulatory elements of the zebrafish raphespecific
gene pet1 and used them to drive expression of an eGFP transgene in the raphe population of serotonergic neurons. With this approach together with retrograde tracing we 1) describe in detail the development, anatomical organization, and projection pattern of zebrafish pet1-positive neurons compared with their mammalian counterparts, 2) identify a new serotonergic population in the ventrolateral zebrafish hindbrain, and 3) reveal some extent of functional subdivisions within the zebrafish superior raphe complex. Together, our results reveal for the first time the specific innervation pattern of the zebrafish raphe and, thus, provide a new model and various tools to investigate further the role of raphe serotonergic neurons in vertebrates.
Mechanisms of metabotropic glutamate receptor-mediated synaptic signalling in cerebellar Purkinje cells
28.10.2008
The metabotropic glutamate receptors type 1 (mGluR1s) are required for a normal function of the mammalian cerebellum. These G-protein-coupled receptors are abundantly expressed in the principle cerebellar cells, namely the Purkinje neurones. Under physiological conditions, mGluR1s are activated
during repetitive activity of both afferent glutamatergic synaptic inputs provided by the climbing and parallel fibres respectively. Unlike the common ionotropic glutamate receptors that underlie rapid synaptic excitation, mGluR1s produce a complex post-synaptic response consisting of a Ca2+-release signal from intracellular stores and a slow excitatory postsynaptic
potential. While it is well established that the mGluR1-dependent
Ca2+-release signal from intracellular stores involves the activation of inositol- trisphosphate receptors, the mechanisms underlying the slow synaptic excitation remained unclear. Here we will review recent evidence indicating an essential role of C-type transient receptor potential (TRPC) cation
channels, especially that of the subunit TRPC3, for the generation of the mGluR1-dependent synaptic current. For the signalling pathways underlying both, Ca2+-release from intracellular stores and the slow synaptic potential,
we present current knowledge about the activators, downstream effectors and possible roles for mGluR1-dependent signalling in Purkinje neurones.
Comparative Analysis of Serotonin Receptor (HTR1A/HTR1B Families) and Transporter (slc6a4a/b) Gene Expression in the Zebrafish Brain
06.10.2008
The Journal of Comparative Neurology,
2008,
511,
512-42
published on 06.10.2008
The Journal of Comparative Neurology, online article
The Journal of Comparative Neurology, online article
In this study we analyze 5-hydroxytryptamine [5-HT]; serotonin) signaling in zebrafish, an increasingly popular vertebrate disease model. We compare and contrast expression of the 5-HT
transporter genes slc6a4a and slc6a4b, which identify 5-HT-producing neurons and three novel 5-HT receptors, htr1aa, htr1ab, and htr1bd. slc6a4a and slc6a4b are expressed in the raphe nuclei, retina, medulla oblongata, paraventricular organ, pretectal diencephalic complex, and caudal zone of the periventricular hypothalamus, in line with the expression profiles of homologues from other vertebrates. Our analysis of serotonin transporter (SERT)-encoding genes also identifies parallel genetic pathways used to build the 5-HT system in zebrafish. In cells in which 5-HT is synthesized by tph1, slc6a4b is used for re-uptake, whereas tph2-positive cells utilize slc6a4a. The receptors htr1aa, htr1ab, and htr1bd also show widespread expression in both the
larval and adult brain. Receptor expression is seen in the superior raphe nucleus, retina, ventral telencephalon, optic tectum, thalamus, posterior tuberculum, cerebellum, hypothalamus, and
reticular formation, thus implicating 5-HT signaling in several neural circuits. We also examine larval brains double-labeled with 5-HTergic and dopaminergic pathway-specific antibodies, to
uncover the identity of some 5-HTergic target neurons. Furthermore, comparison of the expression of transporter and receptor genes also allows us to map sites of autoreceptor activity within the brain. We detect autoreceptor activity in the pretectal diencephalic cluster (htr1aa-, htr1ab-, htr1bd-, and slc6a4a-positive), superior raphe nucleus (htr1aa-, htr1ab-, and slc6a4a-positive), paraventricular organ (htr1aa-, htr1ab-, htr1bd-, and slc6a4b-positive), and the caudal zone of the periventricular hypothalamus (htr1ab- and slc6a4b-positive).
Progeny of Olig2-Expressing Progenitors in the Gray and White Matter of the Adult Mouse Cerebral Cortex
01.10.2008
The Journal of Neuroscience,
2008,
28(41),
10434–10442, doi:10.1523/JNEUROSCI.2831-08.2008
published on 01.10.2008
The Journal of Neuroscience, online article
The Journal of Neuroscience, online article
Despite their abundance, still little is known about the rather frequent, constantly proliferating progenitors spread throughout the adult mouse brain parenchyma. The majority of these progenitors express the basic-helix-loop-helix transcription factor Olig2, and their number further increases after injury. Here, we examine the progeny of this progenitor population by genetic fate mapping using tamoxifen-inducible Cre-recombination in the Olig2 locus to turn on permanent reporter gene expression in the adult brain. Consistent with Olig2 expression in proliferating NG2+ progenitors, most reporter+ cells seen shortly after initiating recombination at adult stages incorporated BrdU and contained the proteoglycan NG2 in both the gray (GM) and the white matter (WM) of the cerebral cortex. However, at longer time points after induction, we observed profound differences in the identity of reporter+ cells in the WM and GM. Whereas most of the Olig2+ progenitors had generated mature, myelinating oligodendrocytes in the WM, hardly any reporter+ cells showing mature oligodendrocyte characteristics were detectable even up to 6 months after recombination in the GM. In the GM, most reporter+ cells remained NG2+, even after injury, but stopped proliferating rather soon after recombination. Thus, our results demonstrate the continuous generation of mature, myelinating oligodendrocytes in the WM, whereas cells in the GM generated mostly postmitotic NG2+ glia.
Clusters of Hyperactive Neurons Near Amyloid Plaques in a Mouse Model of Alzheimer’s Disease
19.09.2008
The neurodegeneration observed in Alzheimer’s disease has been associated with synaptic dismantling and progressive decrease in neuronal activity. We tested this hypothesis in vivo by using two-photon Ca2+ imaging in a mouse model of Alzheimer’s disease. Although a decrease in neuronal activity was seen in 29% of layer 2/3 cortical neurons, 21% of neurons displayed an unexpected increase in the frequency of spontaneous Ca2+ transients. These “hyperactive” neurons were found exclusively near the plaques of amyloid b–depositing mice. The hyperactivity appeared to be due to a relative decrease in synaptic inhibition. Thus, we suggest that a redistribution of synaptic drive between silent and hyperactive neurons, rather than an overall decrease in synaptic activity, provides a mechanism for the disturbed cortical function in Alzheimer’s disease.
Intramembrane Proteolysis of GxGD-type Aspartyl Proteases is slowed by a Familial Alzheimer Disease-like Mutation
03.09.2008
More than one hundred and fifty familial Alzheimer's Disease (FAD)-associated missense mutations in presenilins (PS1 and PS2), the catalytic subunit of the γ- secretase complex, cause aberrant Amyloid β-peptide (Aβ) production, by increasing the relative production of the highly amyloidogenic 42 amino acid
variant. The molecular mechanism behind this pathological activity is unclear and different possibilities ranging from a gain of function to a loss of function have been discussed. γ-Secretase, signal peptide peptidase (SPP) and SPP-like proteases (SPPLs) belong to the same family of GxGD-type intramembrane cleaving aspartyl
proteases and share several functional similarities. We have introduced the FAD-associated PS1 G384A mutation, which occurs within the highly conserved GxGD motif of PS1 right next to the catalytically critical aspartate residue, into the corresponding GxGD
motif of the signal peptide peptidase-like 2b (SPPL2b). Compared to wt SPPL2b, mutant SPPL2b slowed intramembrane proteolysis of tumor necrosis factor α (TNFα) and cause a relative increase of
longer intracellular cleavage products.Since the N-termini of the secreted counterparts remain unchanged, the mutation selectively affects the liberation of the intracellular processing products. In vitro experiments demonstrate that the apparent accumulation of longer intracellular cleavage products is the result of slowed sequential intramembrane cleavage. The longer cleavage products are still converted to shorter peptides, however only after
prolonged incubation time. This suggests that FAD-associated PS
mutation may also result in reduced intramembrane cleavage of β-Amyloid Precursor Protein (βAPP). Indeed, in vitro experiments demonstrate slowed intramembrane proteolysis by γ-secretase containing PS1 with the G384A mutation. As compared to wt PS1, the mutation selectively slowed Aβ40 production, while Aβ42 generation remained unaffected. Thus, the PS1 G384A mutation causes a selective loss of function by slowing the processing
pathway leading to the benign Aβ40.
Lysosomal Activity Associated with Developmental Axon Pruning
03.09.2008
Clearance of cellular debris is a critical feature of the developing nervous system, as evidenced by the severe neurological consequences of lysosomal storage diseases in children. An important developmental process, which generates considerable cellular debris, is synapse elimination, in which many axonal branches are pruned. The fate of these pruned branches is not known. Here, we investigate the role of lysosomal activity in neurons and glia in the removal of axon branches during early postnatal life. Using a probe for lysosomal activity, we observed robust staining associated with retreating motor axons. Lysosomal function was involved in axon removal because retreating axons were cleared more slowly in a mouse model of a lysosomal storage disease. In addition, we found lysosomal activity in the cerebellum at the time of, and at sites where, climbing fibers are eliminated. We propose that lysosomal activity is a central feature of synapse elimination. Moreover, staining for lysosomal activity may serve as a marker for regions of the developing nervous system undergoing axon pruning.
Gsk3β/PKA and Gli1 regulate the maintenance of neural progenitors at the midbrain-hindbrain boundary in concert with E(Spl) factor activity
25.08.2008
Neuronal production in the midbrain-hindbrain domain (MH) of the vertebrate embryonic neural tube depends on a progenitor pool called the ‘intervening zone’ (IZ), located at the midbrain-hindbrain boundary. The progressive recruitment of IZ progenitors along the mediolateral (future dorsoventral) axis prefigures the earlier maturation of the MH basal plate. It also correlates with a lower sensitivity of medial versus lateral IZ progenitors to the neurogenesis inhibition process that maintains the IZ pool. This role is performed in zebrafish by the E(Spl) factors Her5 and Her11, but the molecular cascades cooperating with Her5/11, and those accounting for their reduced effect in the medial IZ, remain unknown. We demonstrate here that the kinases Gsk3β and cAMPdependent protein kinase A (PKA) are novel determinants of IZ formation and cooperate with E(Spl) activity in a dose-dependent
manner. Similar to E(Spl), we show that the activity of Gsk3β/PKA is sensed differently by medial versus lateral IZ progenitors. Furthermore, we identify the transcription factor Gli1, expressed in medial IZ cells, as an antagonist of E(Spl) and Gsk3β/PKA, and
demonstrate that the neurogenesis-promoting activity of Gli1 accounts for the reduced sensitivity of medial IZ progenitors to
neurogenesis inhibitors and their increased propensity to differentiate. We also show that the expression and activity of Gli1 in this process are, surprisingly, independent of Hedgehog signaling. Together, our results suggest a model in which the modulation of E(Spl) and Gsk3β/PKA activities by Gli1 underlies the dynamic properties of IZ maintenance and recruitment.
TRPC3 Channels Are Required for Synaptic Transmission and Motor Coordination
14.08.2008
In the mammalian central nervous system, slow synaptic excitation involves the activation of metabotropic glutamate receptors (mGluRs). It has been proposed that C1-type transient receptor potential (TRPC1) channels underlie this synaptic excitation, but our analysis of TRPC1-deficient mice does not support this hypothesis. Here, we show unambiguously that it is TRPC3 that is needed for mGluRdependent synaptic signaling in mouse cerebellar Purkinje cells. TRPC3 is the most abundantly expressed TRPC subunit in Purkinje cells. In mutant mice lacking TRPC3, both slow synaptic potentials and mGluR-mediated inward currents are completely
absent, while the synaptically mediated Ca2+ release signals from intracellular stores are unchanged. Importantly, TRPC3 knockout mice exhibit an impaired walking behavior. Taken together, our results establish TRPC3 as a new type of postsynaptic channel
that mediates mGluR-dependent synaptic transmission in cerebellar Purkinje cells and is crucial for motor coordination.
Fgf signaling in the zebrafish adult brain: Association of Fgf activity with ventricular zones but not cell proliferation
29.07.2008
JCN,
2008,
510 No.4,
422-439
published on 29.07.2008
The Journal of Comparative Neurology, online article
The Journal of Comparative Neurology, online article
The zebrafish adult brain contains numerous neural progenitors and is a good model to approach the general mechanisms of adult neural stem cell maintenance and neurogenesis. Here we use this model to test for a correlation between Fgf signaling and cell proliferation in adult progenitor zones. We report expression of Fgf signals (fgf3,4,8a,8b,17b), receptors (fgfr1-4), and targets (erm, pea3, dusp6, spry1,2,4, and P-ERK) and document that genes of the embryonic fgf8 synexpression group acquire strikingly divergent patterns in the adult brain. We further document the specific expression of fgf3, fgfr1-3, dusp6, and P-ERK in ventricular zones, which contain neural progenitors. In these locations, however, a comparison at the single-cell level of fgfr/P-ERK expression with bromo-deoxy-uridine (BrdU) incorporation and the proliferation marker MCM5 indicates that Fgf signaling is not specifically associated with proliferating progenitors. Rather, it correlates with the ventricular radial glia state, some of which only are progenitors. Together these results stress the importance of Fgf signaling in the adult brain and establish the basis to study its function in zebrafish, in particular in relation to adult neurogenesis.
Intramembrane Proteolysis by gamma-Secretase
23.07.2008
Journal of Biological Chemistry,
2008,
DOI 10.1074/jbc.R800010200,
published on 23.07.2008
www.jbc.org, online article
www.jbc.org, online article
Gamma-Secretase mediates the final proteolytic cleavage, which liberates Amyloid beta -peptide, the major component of senile plaques in the brain of Alzheimer's disease patients. Therefore gamma -secretase is a prime target for Amyloid beta -peptide lowering therapeutic strategies. gamma -Secretase is a protein complex composed of four different subunits, presenilin, APH-1, NCT and PEN-2, which are most likely present in a 1:1:1:1 stoichiometry. Presenilin harbors the catalytically active site, which is critically required for the aspartyl protease activity of gamma -secretase. Moreover, numerous familial Alzheimer's disease associated mutations within the presenilins increase the production of the aggregation prone and neurotoxic 42 amino acid amyloid beta -peptide. NCT may serve as a substrate receptor, although this has recently been challenged. PEN-2 is required to stabilize presenilin within the gamma -secretase complex. No particular function has so far been assigned to APH-1. The four components are sufficient and required for gamma -secretase activity. At least six different gamma -secretase complexes exist which are composed of different variants of presenilin and APH-1. All gamma -secretase complexes can exert pathological amyloid beta -peptide production. Assembly of the gamma -secretase complex occurs within the endoplasmic reticulum and only fully assembled and functional gamma -secretase complexes are transported to the plasma membrane. Structural analysis by electron microscopy and chemical cross-linking reveals a water-containing cavity, which allows intramembrane proteolysis. Specific and highly sensitive gamma -secretase inhibitors have been developed, however they interfere with the physiological function of gamma -secretase in Notch-signaling and thus cause rather significant side effects in human trials. Modulators of gamma -secretase, which selectively affect the production of the pathological 42 amino acid amyloid beta -peptide do not inhibit Notch signaling.
Regulatory RNA goes awry in Alzheimer’s disease
01.07.2008
In the current issue of Nature Medicine, Faghihi et al. report that a noncoding antisense RNA against β-secretase, also known as BACE1 (β-site amyloid precursor protein (APP)-cleaving enzyme), may contribute to pathogenesis of Alzheimer's disease. BACE1 is a vertebrate-specific enzyme, which, together with presenilin-dependent γ-secretase, cleaves APP to generate the neurotoxic amyloid-β peptide (Aβ).
MicroRNA-9 directs late organizer activity of the midbrain-hindbrain boundary
10.11.2008
Nature Neuroscience,
2008,
11 No.6,
641-8
published on 04.05.2008
Nature Neuroscience, online article
Nature Neuroscience, online article
Serotonin is a major central nervous modulator of physiology
and behavior and plays fundamental roles during development
and plasticity of the vertebrate central nervous system
(CNS). Understanding the developmental control and
functions of serotonergic neurons is therefore an important
task. In all vertebrates, prominent serotonergic neurons are
found in the superior and inferior raphe nuclei in the hindbrain
innervating most CNS regions. In addition, all vertebrates
except for mammals harbor other serotonergic centers,
including several populations in the diencephalon. This, in combination with the intricate and wide distribution
of serotonergic fibers, makes it difficult to sort out serotonergic
innervation originating from the raphe from that of
other serotonergic cell populations. To resolve this issue,
we isolated the regulatory elements of the zebrafish raphespecific
gene pet1 and used them to drive expression of an eGFP transgene in the raphe population of serotonergic neurons. With this approach together with retrograde tracing we 1) describe in detail the development, anatomical organization, and projection pattern of zebrafish pet1-positive neurons compared with their mammalian counterparts, 2) identify a new serotonergic population in the ventrolateral zebrafish hindbrain, and 3) reveal some extent of functional subdivisions within the zebrafish superior raphe complex. Together, our results reveal for the first time the specific innervation pattern of the zebrafish raphe and, thus, provide a new model and various tools to investigate further the role of raphe serotonergic neurons in vertebrates.
Development of Presynaptic Inhibition Onto Retinal Bipolar Cell Axon Terminals Is Subclass-Specific
24.04.2008
Synaptic integration is modulated by inhibition onto the dendrites of postsynaptic cells. However, presynaptic inhibition at axonal terminals also plays a critical role in the regulation of neurotransmission. In contrast to the development of inhibitory synapses onto dendrites, GABAergic/glycinergic synaptogenesis onto axon terminals has not been widely studied. Because retinal bipolar cells receive subclass-specific patterns of GABAergic and glycinergic presynaptic inhibition, they are a good model for studying the development of inhibition at axon terminals.
Here, using whole cell recording methods and transgenic mice in
which subclasses of retinal bipolar cells are labeled, we determined the temporal sequence and patterning of functional GABAergic and glycinergic input onto the major subclasses of bipolar cells. We found that the maturation of GABAergic and glycinergic synapses onto the axons of rod bipolar cells (RBCs), ON-cone bipolar cells (ON-CBCs) and OFF-cone bipolar cells (OFF-CBCs) were temporally distinct: spontaneous chloride-mediated currents are present in RBCs earlier in development compared with ON- and OFF-CBC, and RBCs receive
GABAergic and glycinergic input simultaneously, whereas in OFFCBCs, glycinergic transmission emerges before GABAergic transmission. Because ON-CBCs show little inhibitory activity, GABAergic and glycinergic events could not be pharmacologically distinguished for these bipolar cells. The balance of GABAergic and glycinergic input that is unique to RBCs and OFF-CBCs is established shortly after the onset of synapse formation and precedes visual experience. Our data suggest that presynaptic modulation of glutamate transmission from bipolar cells matures rapidly and is differentially coordinated for GABAergic and glycinergic synapses onto distinct bipolar cell subclasses.
A Novel Sorting Nexin Modulates Endocytic Trafficking and alpha-Secretase Cleavage of the Amyloid Precursor Protein
19.03.2008
The Jounal of Biological Chemistry,
2008,
DOI: 10.1074/jbc.M801531200,
published on 19.03.2008
www.jbc.org, online article
www.jbc.org, online article
Ectodomain shedding of the amyloid precursor protein (APP) by the two proteases alpha- and ß-secretase is a key regulatory event in the generation of the Alzheimer’s disease amyloid ß peptide (Aß). ß-secretase catalyzes the first step in Aß-generation, whereas a-secretase cleaves within the Aß domain, prevents Aß generation and generates a secreted form of APP with neuroprotective properties. At present, little is known about the cellular mechanisms that control APP alpha-secretase cleavage and Aß generation. To explore the contributory pathways, we carried out an expression cloning screen. We identified a novel member of the sorting nexin (SNX) family of endosomal trafficking proteins, called SNX33, as a new activator of APP alpha-secretase cleavage. SNX33 is a homolog of SNX9 and was found to be a ubiquitously expressed phospho-protein. Exogenous expression of SNX33 in cultured cells increased APP alpha-secretase cleavage four-fold, but surprisingly had little effect on ß-secretase cleavage. This effect was similar to the expression of the dominant-negative dynamin 1 mutant K44A. SNX33 bound the endocytic GTPase dynamin and reduced the rate of APP endocytosis in a dynamin-dependent manner. This led to an increase of APP at the plasma membrane, where a-secretase cleavage mostly occurs. In summary, our study identifies SNX33 as a new endocytic protein, which modulates APP endocytosis and APP a-secretase cleavage, and demonstrates that the rate of APP endocytosis is a major control factor for APP alpha-secretase cleavage.
The two faces of protein misfolding: gain- and loss-of-function in neurodegenerative diseases
01.03.2008
The etiologies of neurodegenerative diseases may be diverse; however, a common pathological denominator is the formation of aberrant protein conformers and the occurrence of pathognomonic proteinaceous deposits. Different approaches coming from neuropathology, genetics, animal modeling and biophysics have established a crucial role of protein misfolding in the pathogenic
process. However, there is an ongoing debate about the nature of the harmful proteinaceous species and how toxic conformers selectively damage neuronal populations. Increasing evidence indicates that soluble oligomers are associated with early pathological alterations, and strikingly, oligomeric assemblies of different disease-associated proteins may share common structural features. A major step towards the understanding of mechanisms implicated in neuronal degeneration is the identification of genes, which are responsible for familial variants of neurodegenerative diseases. Studies based on these disease-associated genes illuminated the two faces of protein misfolding in neurodegeneration: a gain of toxic function and a loss of physiological function, which can even occur in combination. Here, we summarize how these two faces of
protein misfolding contribute to the pathomechanisms of Alzheimer’s disease, frontotemporal lobar degeneration, Parkinson’s disease and prion diseases.
Prospective isolation of functionally distinct radial glial subtypes—Lineage and transcriptome analysis
01.02.2008
MCN,
2008,
doi:10.1016/j.mcn.2008.01.012,
published on 01.02.2008
Molecular and Cellular Neuroscience, online article
Molecular and Cellular Neuroscience, online article
Since the discovery of radial glia as the source of neurons, their
heterogeneity in regard to neurogenesis has been described by clonal and time-lapse analysis in vitro. However, the molecular determinants specifying neurogenic radial glia differently from radial glia that mostly self-renew remain ill-defined. Here, we isolated two radial glial subsets that co-exist at mid-neurogenesis in the developing cerebral cortex and their immediate progeny. While one subset generates neurons directly, the other is largely on-neurogenic but also gives rise to Tbr2-positive basal precursors, thereby contributing indirectly to neurogenesis. Isolation of these distinct radial glia subtypes allowed determining interesting differences in their transcriptome. These transcriptomes were also strikingly different from the transcriptome of radial glia isolated at the end of neurogenesis. This analysis therefore identifies, for the first time, the lineage origin of basal progenitors and the molecular differences of this lineage in comparison to directly neurogenic and gliogenic radial glia.
Homosynaptic Long-Term Synaptic Potentiation of the "Winner" Climbing Fiber Synapse in developing Purkinje Cells
23.01.2008
J. Neurosci.,
2008,
28 (4),
798-807
published on 23.01.2008
The Journal of Neuroscience, online article
The Journal of Neuroscience, online article
During the developmental formation of neuronal circuits, redundant synapses are eliminated and persisting synapses strengthened. In
the immature cerebellum, climbing fiber–Purkinje cell synapses undergo a pronounced synaptic rewiring, from a multiple innervation
around birth to a mono-innervation in adults. An early stage of this process consists in the differentiation of initially equally strong
synapses into one “large” and several “small” synaptic inputs. By performing whole-cell recordings in Purkinje cells of rat cerebellar
slices, we found that the coincident activation of a Purkinje cell and one of its afferent climbing fibers induces homosynaptic long-term
synaptic potentiation (LTP). This LTP requires postsynaptic Ca2+ signaling and involves an increase in the single channel conductance
of the postsynaptic AMPA receptors. Interestingly, LTP occurs exclusively at large synaptic inputs. It is not observed at small inputs that are eventually eliminated. Thus, we identified a new form of LTP that is expressed uniquely and just for a restricted period of early
development in the large climbing fiber inputs. Our results suggest that this LTP mediates the activity-dependent maturation of the
“winner” climbing fiber.
Adult Neurogenesis Requires Smad4-Mediated Bone Morphogenic Protein Signaling in Stem Cells
09.01.2008
In the mammalian brain, neurogenesis continues only in few regions of the forebrain. The molecular signals governing neurogenesis in
these unique neurogenic niches, however, are still ill defined. Here, we show that bone morphogenic protein (BMP)-mediated signaling
is active in adult neural stem cells and is crucial to initiate the neurogenic lineage in the adult mouse subependymal zone. Conditional deletion of Smad4 in adult neural stem cells severely impairs neurogenesis, and this is phenocopied by infusion of Noggin, an extracellular antagonist of BMP. Smad4 deletion in stem, but not progenitor cells, as well as Noggin infusion lead to an increased number of Olig2-expressing progeny that migrate to the corpus callosum and differentiate into oligodendrocytes. Transplantation experiments further verified the cell-autonomous nature of this phenotype. Thus, BMP-mediated signaling via Smad4 is required to initiate neurogenesis from adult neural stem cells and suppress the alternative fate of oligodendrogliogenesis.
