Biomedical subjects

Alaa El-Husseini

Publications and source records attributed to Alaa El-Husseini.

18 recordsLinked to original sources

PubMed2006

Recent advances in basic neurosciences and brain disease: from synapses to behavior.

Understanding basic neuronal mechanisms hold the hope for future treatment of brain disease. The 1st international conference on synapse, memory, drug addiction and pain was held in beautiful downtown Toronto, Canada on August 21-23, 2006. Unlike other traditional conferences, this new meeting focused on three major aims: (1) to promote new and cutting edge research in neuroscience; (2) to encourage international information exchange and scientific collaborations; and (3) to provide a platform for active scientists to discuss new findings. Up to 64 investigators presented their recent discoveries, from basic synaptic mechanisms to genes related to human brain disease. This meeting was in part sponsored by Molecular Pain, together with University of Toronto (Faculty of Medicine, Department of Physiology as well as Center for the Study of Pain). Our goal for this meeting is to promote future active scientific collaborations and improve human health through fundamental basic neuroscience researches. The second international meeting on Neurons and Brain Disease will be held in Toronto (August 29-31, 2007).

Behavior↗

PubMed2006

NIPA1(SPG6), the basis for autosomal dominant form of hereditary spastic paraplegia, encodes a functional Mg2+ transporter.

Mutations in the NIPA1(SPG6) gene, named for "nonimprinted in Prader-Willi/Angelman" has been implicated in one form of autosomal dominant hereditary spastic paraplegia (HSP), a neurodegenerative disorder characterized by progressive lower limb spasticity and weakness. However, the function of NIPA1 is unknown. Here, we show that reduced magnesium concentration enhances expression of NIPA1 suggesting a role in cellular magnesium metabolism. Indeed NIPA1 mediates Mg2+ uptake that is electrogenic, voltage-dependent, and saturable with a Michaelis constant of 0.69+/-0.21 mM when expressed in Xenopus oocytes. Subcellular localization with immunofluorescence showed that endogenous NIPA1 protein associates with early endosomes and the cell surface in a variety of neuronal and epithelial cells. As expected of a magnesium-responsive gene, we find that altered magnesium concentration leads to a redistribution between the endosomal compartment and the plasma membrane; high magnesium results in diminished cell surface NIPA1 whereas low magnesium leads to accumulation in early endosomes and recruitment to the plasma membrane. The mouse NIPA1 mutants, T39R and G100R, corresponding to the respective human mutants showed a loss-of-function when expressed in oocytes and altered trafficking in transfected COS7 cells. We conclude that NIPA1 normally encodes a Mg2+ transporter and the loss-of function of NIPA1(SPG6) due to abnormal trafficking of the mutated protein provides the basis of the HSP phenotype.

Amino Acid Sequence↗

PubMed2006

Cell adhesion molecules at the synapse.

Synapses are specialized intercellular junctions whose specificity and plasticity provide the structural and functional basis for the formation and maintenance of the complex neural network in the brain. The number, location, and type of synapses formed are well controlled, since synaptic circuits are formed in a highly reproducible way. This implies the existence of cellular and molecular properties that determine the connectivity of each neuron in the nervous system. Recent evidence has elucidated that these key features of the synapse are regulated by several families of cell-adhesion molecules (CAMs) enriched at synaptic junctions, including neuroligins, SynCAM, NCAM, L1-CAM, cadherins, protocadherins, and integrins. In this review we will discuss the various stages of synaptogenesis from the perspective of CAMs: Contact initiation, recruitment of presynaptic and postsynaptic proteins, synapse maturation/stabilization or elimination, and synaptic plasticity. We will also highlight some of the factors that regulate the function of these CAMs at the synapse, and discuss how dysfunction of these adhesive systems may contribute to several neurological disorders.

Cell Adhesion Molecules↗

PubMed2006

Palmitoylation of huntingtin by HIP14 is essential for its trafficking and function.

Post-translational modification by the lipid palmitate is crucial for the correct targeting and function of many proteins. Here we show that huntingtin (htt) is normally palmitoylated at cysteine 214, which is essential for its trafficking and function. The palmitoylation and distribution of htt are regulated by the palmitoyl transferase huntingtin interacting protein 14 (HIP14). Expansion of the polyglutamine tract of htt, which causes Huntington disease, results in reduced interaction between mutant htt and HIP14 and consequently in a marked reduction in palmitoylation. Mutation of the palmitoylation site of htt, making it palmitoylation resistant, accelerates inclusion formation and increases neuronal toxicity. Downregulation of HIP14 in mouse neurons expressing wild-type and mutant htt increases inclusion formation, whereas overexpression of HIP14 substantially reduces inclusions. These results suggest that the expansion of the polyglutamine tract in htt results in decreased palmitoylation, which contributes to the formation of inclusion bodies and enhanced neuronal toxicity.

Acyltransferases↗

PubMed2006

Interleukin-1 beta modulates AMPA receptor expression and phosphorylation in hippocampal neurons.

Interleukin (IL)-1beta is a pro-inflammatory cytokine involved in modulating inflammation and stress responses in the brain. Central administration of IL-1beta impairs both memory functions and long-term potentiation (LTP) induction. However, the molecular events responsible for the downstream effects of IL-1beta are not fully understood. Given the potential regulatory role of IL-1beta in LTP, we assessed whether IL-1beta influences surface expression and phosphorylation of glutamate receptors. We found that IL-1beta, but not IL-10 or tumour necrosis factor (TNF)-alpha, down-regulated the surface expression and Ser831 phosphorylation of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor subunit GluR1. Agents that block IL-1beta receptor activity abolished these effects. In contrast, no change in the surface expression of the N-methyl-d-aspartate (NMDA) receptor subunit NR1 was observed. The inhibition of NMDA receptor activity or depletion of extracellular calcium blocked IL-1beta effects on GluR1 phosphorylation and surface expression. NMDA-mediated calcium influx was also regulated by IL-1beta. These findings suggest that IL-1beta selectively regulates AMPA receptor phosphorylation and surface expression through extracellular calcium and an unknown mechanism involving NMDA receptor activity.

Animals↗

PubMed2006

A preformed complex of postsynaptic proteins is involved in excitatory synapse development.

Nonsynaptic clusters of postsynaptic proteins have been documented; however, their role remains elusive. We monitored the trafficking of several candidate proteins implicated in synaptogenesis, when nonsynaptic clusters of scaffold proteins are most abundant. We find a protein complex consisting of two populations that differ in their content, mobility, and involvement in synapse formation. One subpopulation is mobile and relies on actin transport for delivery to nascent and existing synapses. These mobile clusters contain the scaffolding proteins PSD-95, GKAP, and Shank. A proportion of mobile clusters that exhibits slow movement and travels short distances contains neuroligin-1. The second group consists of stationary nonsynaptic scaffold complexes that mainly contain neuroligin-1, can recruit synaptophysin-containing axonal transport vesicles, and are readily transformed to functional presynaptic contacts that recycle the vital dye FM 4-64. These results postulate a mechanism whereby preformed scaffold protein complexes serve as predetermined postsynaptic hotspots for establishment of new functional excitatory synapses.

Actins↗

PubMed2006

A monovalent streptavidin with a single femtomolar biotin binding site.

Streptavidin and avidin are used ubiquitously because of the remarkable affinity of their biotin binding, but they are tetramers, which disrupts many of their applications. Making either protein monomeric reduces affinity by at least 10(4)-fold because part of the binding site comes from a neighboring subunit. Here we engineered a streptavidin tetramer with only one functional biotin binding subunit that retained the affinity, off rate and thermostability of wild-type streptavidin. In denaturant, we mixed a streptavidin variant containing three mutations that block biotin binding with wild-type streptavidin in a 3:1 ratio. Then we generated monovalent streptavidin by refolding and nickel-affinity purification. Similarly, we purified defined tetramers with two or three biotin binding subunits. Labeling of site-specifically biotinylated neuroligin-1 with monovalent streptavidin allowed stable neuroligin-1 tracking without cross-linking, whereas wild-type streptavidin aggregated neuroligin-1 and disrupted presynaptic contacts. Monovalent streptavidin should find general application in biomolecule labeling, single-particle tracking and nanotechnology.

Animals↗

PubMed2005

Involvement of myosin Vb in glutamate receptor trafficking.

Myosin V motors mediate cargo transport; however, the identity of neuronal molecules transported by these proteins remains unknown. Here we show that myosin Vb is expressed in several neuronal populations and associates with the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate-type glutamate receptor subunit GluR1. In developing hippocampal neurons, expression of the tail domain of myosin Vb, but not myosin Va, enhanced GluR1 accumulation in the soma and reduced its surface expression. These changes were accompanied by reduced GluR1 clustering and diminished frequency of excitatory but not inhibitory synaptic currents. Similar effects were observed upon expression of full-length myosin Vb lacking a C-terminal region required for binding to the small GTPase Rab11. In contrast, mutant myosin Vb did not change the localization of several other neurotransmitter receptors, including the glutamate receptor subunit NR1. These results reveal a novel mechanism for the transport of a specific glutamate receptor subunit in neurons mediated by a member of the myosin V family.

Animals↗

PubMed2005

Building excitatory and inhibitory synapses: balancing neuroligin partnerships.

Processing of neural information is thought to occur by integration of excitatory and inhibitory synaptic inputs. As such, precise control mechanisms must exist to maintain an appropriate balance between each synapse type. Recent findings indicate that neuroligins and their synaptic binding partners modulate the development of both excitatory and inhibitory synapses. Here we highlight these findings and discuss a mechanism potentially involved in controlling the balance between excitation and inhibition.

Animals↗

PubMed2005

New players tip the scales in the balance between excitatory and inhibitory synapses.

Synaptogenesis is a highly controlled process, involving a vast array of players which include cell adhesion molecules, scaffolding and signaling proteins, neurotransmitter receptors and proteins associated with the synaptic vesicle machinery. These molecules cooperate in an intricate manner on both the pre- and postsynaptic sides to orchestrate the precise assembly of neuronal contacts. This is an amazing feat considering that a single neuron receives tens of thousands of synaptic inputs but virtually no mismatch between pre- and postsynaptic components occur in vivo. One crucial aspect of synapse formation is whether a nascent synapse will develop into an excitatory or inhibitory contact. The tight control of a balance between the types of synapses formed regulates the overall neuronal excitability, and is thus critical for normal brain function and plasticity. However, little is known about how this balance is achieved. This review discusses recent findings which provide clues to how neurons may control excitatory and inhibitory synapse formation, with focus on the involvement of the neuroligin family and PSD-95 in this process.

Animals↗

PubMed2005

Neuroligins mediate excitatory and inhibitory synapse formation: involvement of PSD-95 and neurexin-1beta in neuroligin-induced synaptic specificity.

The balance between excitatory and inhibitory synapses is a tightly regulated process that requires differential recruitment of proteins that dictate the specificity of newly formed contacts. However, factors that control this process remain unidentified. Here we show that members of the neuroligin (NLG) family, including NLG1, NLG2, and NLG3, drive the formation of both excitatory and inhibitory presynaptic contacts. The enrichment of endogenous NLG1 at excitatory contacts and NLG2 at inhibitory synapses supports an important in vivo role for these proteins in the development of both types of contacts. Immunocytochemical and electrophysiological analysis showed that the effects on excitatory and inhibitory synapses can be blocked by treatment with a fusion protein containing the extracellular domain of neurexin-1beta. We also found that overexpression of PSD-95, a postsynaptic binding partner of NLGs, resulted in a shift in the distribution of NLG2 from inhibitory to excitatory synapses. These findings reveal a critical role for NLGs and their synaptic partners in controlling the number of inhibitory and excitatory synapses. Furthermore, relative levels of PSD-95 alter the ratio of excitatory to inhibitory synaptic contacts by sequestering members of the NLG family to excitatory synapses.

Animals↗

PubMed2005

Modulation of neuronal protein trafficking and function by palmitoylation.

Modification of proteins with the lipid palmitate regulates targeting to specific vesicular compartments and synaptic membranes. Mounting evidence indicates that this lipid modification modulates diverse aspects of neuronal development and synaptic transmission. In particular, palmitoylation regulates the function of proteins that control neuronal differentiation, axonal pathfinding and filopodia formation. In addition, trafficking of numerous proteins associated with synaptic vesicle release machinery requires protein palmitoylation. Remarkably, reversible palmitoylation of specific scaffolding proteins and signaling molecules dynamically regulates ion channel clustering and synaptic strength. The recent discovery of enzymes that palmitoylate specific subsets of synaptic proteins suggests that this process is tightly controlled in neurons.

Animals↗

PubMed2004

Huntingtin-interacting protein HIP14 is a palmitoyl transferase involved in palmitoylation and trafficking of multiple neuronal proteins.

In neurons, posttranslational modification by palmitate regulates the trafficking and function of signaling molecules, neurotransmitter receptors, and associated synaptic scaffolding proteins. However, the enzymatic machinery involved in protein palmitoylation has remained elusive. Here, using biochemical assays, we show that huntingtin (htt) interacting protein, HIP14, is a neuronal palmitoyl transferase (PAT). HIP14 shows remarkable substrate specificity for neuronal proteins, including SNAP-25, PSD-95, GAD65, synaptotagmin I, and htt. Conversely, HIP14 is catalytically invariant toward paralemmin and synaptotagmin VII. Exogenous HIP14 enhances palmitoylation-dependent vesicular trafficking of several acylated proteins in both heterologous cells and neurons. Moreover, interference with endogenous expression of HIP14 reduces clustering of PSD-95 and GAD65 in neurons. These findings define HIP14 as a mammalian palmitoyl transferase involved in the palmitoylation and trafficking of multiple neuronal proteins.

Acyltransferases↗

PubMed2004

Cell adhesion molecules in synapse formation.

Neuronal transmission relies on signals transmitted through a vast array of excitatory and inhibitory neuronal synaptic connections. How do axons communicate with dendrites to build synapses, and what molecules regulate this interaction? There is a wealth of evidence suggesting that cell adhesion molecules (CAMs) provide much of the information required for synapse formation. This review highlights the molecular mechanisms used by CAMs to regulate presynaptic and postsynaptic differentiation.

Animals↗

PubMed2004

A balance between excitatory and inhibitory synapses is controlled by PSD-95 and neuroligin.

Factors that control differentiation of presynaptic and postsynaptic elements into excitatory or inhibitory synapses are poorly defined. Here we show that the postsynaptic density (PSD) proteins PSD-95 and neuroligin-1 (NLG) are critical for dictating the ratio of excitatory-to-inhibitory synaptic contacts. Exogenous NLG increased both excitatory and inhibitory presynaptic contacts and the frequency of miniature excitatory and inhibitory synaptic currents. In contrast, PSD-95 overexpression enhanced excitatory synapse size and miniature frequency, but reduced the number of inhibitory synaptic contacts. Introduction of PSD-95 with NLG augmented synaptic clustering of NLG and abolished NLG effects on inhibitory synapses. Interfering with endogenous PSD-95 expression alone was sufficient to reduce the ratio of excitatory-to-inhibitory synapses. These findings elucidate a mechanism by which the amounts of specific elements critical for synapse formation control the ratio of excitatory-to-inhibitory synaptic input.

Animals↗

PubMed2004

Presynaptic trafficking of synaptotagmin I is regulated by protein palmitoylation.

Protein palmitoylation plays a critical role in sorting and targeting of several proteins to pre- and postsynaptic sites. In this study, we have analyzed the role of palmitoylation in trafficking of synaptotagmin I and its modulation by synaptic activity. We found that palmitoylation of N-terminal cysteines contributed to sorting of synaptotagmin I to an intracellular vesicular compartment at the presynaptic terminal. Presynaptic targeting is a unique feature of N-terminal sequences of synaptotagmin I because the palmitoylated N terminus of synaptotagmin VII failed to localize to presynaptic sites. We also found that palmitate was stably associated with both synaptotagmin I and SNAP-25 and that rapid neuronal depolarization did not affect palmitate turnover on these proteins. However, long-term treatment with drugs that either block synaptic activity or disrupt SNARE complex assembly modulated palmitoylation and accumulation of synaptotagmin I at presynaptic sites. We conclude that palmitoylation is involved in trafficking of specific elements involved in transmitter release and that distinct mechanisms regulate addition and removal of palmitate on select neuronal proteins.

Animals↗

PubMed2004

Modulation of dopamine mediated phosphorylation of AMPA receptors by PSD-95 and AKAP79/150.

Communication between dopaminergic and glutamatergic synapses is critical for several functions related to cognition and emotion. Here, we examined whether dopamine receptor activity regulates phosphorylation and trafficking of the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor subunit, GluR1. We find treatment with a dopamine D1 receptor agonist enhanced GluR1 phosphorylation at Ser845, the PKA phosphorylation site, in both striatal and prefrontal cortical neurons. Enhanced phosphorylation of GluR1 also correlated with increased amounts of GluR1 on the cell surface. These effects were disrupted by expression of mutant forms of the A-kinase anchoring protein (AKAP79/150) and the postsynaptic density protein, PSD-95, that fail to target synaptic sites. Similar enhancement of the phosphorylation of GluR1 was observed in the nucleus accumbens upon stimulation of dopamine release in vivo using electrical stimulation of dopamine cell bodies in the ventral tegmental area. These results suggest in vivo stimulation of dopamine release directly influences AMPA receptor phosphorylation and together with in vitro data indicate that coupling of the AMPA receptor to AKAP79/150 and PSD-95 modulate this process.

2,3,4,5-Tetrahydro-7,8-dihydroxy-1-phenyl-1H-3-ben↗

PubMed2003

Stargazin differentially controls the trafficking of alpha-amino-3-hydroxyl-5-methyl-4-isoxazolepropionate and kainate receptors.

Synaptic plasticity at excitatory synapses in the brain is largely achieved by rapid changes in the number of synaptic alpha-amino-3-hydroxyl-5-methyl-4-isoxazolepropionate (AMPA) receptors. Stargazin, a membrane protein that interacts with AMPA receptors, is believed to play a pivotal role in trafficking AMPA receptors to the plasma membrane and targeting them to the synapse. However, it is unclear whether the trafficking of kainate receptors, which are structurally very similar to AMPA receptors, is also dependent on stargazin. Here we show that in both cerebellar granule cells and in Xenopus laevis oocytes expression system, surface delivery of kainate receptor is independent of stargazin. These results suggest that stargazin action is highly selective for AMPA receptors.

Animals↗