Dynamic regulation of AMPA-type glutamate receptors represents a primary mechanism for controlling synaptic strength, though mechanisms for this process are poorly understood. The palmitoylated postsynaptic density protein, PSD-95, regulates synaptic plasticity and associates with the AMPA receptor trafficking protein, stargazin. Here, we identify palmitate cycling on PSD-95 at the synapse and find that palmitate turnover on PSD-95 is regulated by glutamate receptor activity. Acutely blocking palmitoylation disperses synaptic clusters of PSD-95 and causes a selective loss of synaptic AMPA receptors. We also find that rapid glutamate-mediated AMPA receptor internalization requires depalmitoylation of PSD-95. In a nonneuronal model system, clustering of PSD-95, stargazin, and AMPA receptors is also regulated by ongoing palmitoylation of PSD-95 at the plasma membrane. These studies suggest that palmitate cycling on PSD-95 can regulate synaptic strength and regulates aspects of activity-dependent plasticity.
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.
Although neuronal axons and dendrites with their associated filopodia and spines exhibit a profound cell polarity, the mechanism by which they develop is largely unknown. Here, we demonstrate that specific palmitoylated protein motifs, characterized by two adjacent cysteines and nearby basic residues, are sufficient to induce filopodial extensions in heterologous cells and to increase the number of filopodia and the branching of dendrites and axons in neurons. Such motifs are present at the N-terminus of GAP-43 and the C-terminus of paralemmin, two neuronal proteins implicated in cytoskeletal organization and filopodial outgrowth. Filopodia induction is blocked by mutations of the palmitoylated sites or by treatment with 2-bromopalmitate, an agent that inhibits protein palmitoylation. Moreover, overexpression of a constitutively active form of ARF6, a GTPase that regulates membrane cycling and dendritic branching reversed the effects of the acylated protein motifs. Filopodia induction by the specific palmitoylated motifs was also reduced upon overexpression of a dominant negative form of the GTPase cdc42. These results demonstrate that select dually lipidated protein motifs trigger changes in the development and growth of neuronal processes. INTRODUCTIONNeurons possess an elaborate plasma membrane architecture that includes axons, dendrites, and synaptic sites (Da Silva and Dotti, 2002). Modulation of the plasma membrane shape and composition regulate processes outgrowth, axonal development, dendritic branching, and the construction of synapses (Jontes and Smith, 2000). These changes are regulated by interactions between the constituents of the plasma membrane, the exo/endocytic vesicles, and the cytoskeleton (Wood and Martin, 2002). In nonneuronal cells, differential modulation of membrane flow can result in the formation of processes such as microspikes, lamellopodia, and filopodia (Wood and Martin, 2002). However, factors that regulate the formation and maintenance of specific classes of membranous processes in neuronal cells remain poorly understood.Dendritic filopodia, cell surface extensions filled with tight parallel bundles of actin filaments, are thought to be precursors for developing synapses (Dailey and Smith, 1996;Small, 1988). Filopodia are typically 5-35 m in length and rapidly alter their position and shape. In developing axons, filopodia occur at the tips of growth cones and may aid the axon in finding its appropriate target. In dendrites, filopodia act as precursors to spines, short bulbous protrusions of the dendrite that form excitatory postsynaptic contacts on many neurons (Jontes and Smith, 2000). Thus, changes in membrane structure that result in the formation of filopodia are likely important in axonal guidance and spine formation. In contrast, dendritic branching is thought to be mediated via an actin and/or microtubule-dependent mechanisms (Jan and Jan, 2003). Recent studies showed that in early neuronal development dendrites originate from lamellopodia and short neurites (Jan and Jan, ...
Dendritic filopodia are thought to participate in neuronal contact formation and development of dendritic spines; however, molecules that regulate filopodia extension and their maturation to spines remain largely unknown. Here we identify paralemmin-1 as a regulator of filopodia induction and spine maturation. Paralemmin-1 localizes to dendritic membranes, and its ability to induce filopodia and recruit synaptic elements to contact sites requires protein acylation. Effects of paralemmin-1 on synapse maturation are modulated by alternative splicing that regulates spine formation and recruitment of AMPA-type glutamate receptors. Paralemmin-1 enrichment at the plasma membrane is subject to rapid changes in neuronal excitability, and this process controls neuronal activity-driven effects on protrusion expansion. Knockdown of paralemmin-1 in developing neurons reduces the number of filopodia and spines formed and diminishes the effects of Shank1b on the transformation of existing filopodia into spines. Our study identifies a key role for paralemmin-1 in spine maturation through modulation of filopodia induction. INTRODUCTIONDuring CNS excitatory synapse development, the formation of spines, bulbous protrusions enriched with F-actin, is essential for proper synaptic transmission and neuronal function (Hall and Nobes, 2000;Yuste and Bonhoeffer, 2004;Halpain et al., 2005;Matus, 2005;. Spines contain a plethora of proteins including neurotransmitter receptors, cytoskeleton-associated proteins, and cell adhesion molecules. Spines are pleomorphic protrusions that can be modified by changes in neuronal activity, which regulate actin-based motility (Fischer et al., 1998;Portera-Cailliau et al., 2003;Matus, 2005). Defects in spine maturation and function have been associated with several forms of mental retardation including Down, Rett, Fragile X, and fetal alcohol syndromes. Some of these disorders exhibit a reduction in spine size and density and the formation of long, thin filopodia-like structures (Hering and Sheng, 2001;Zoghbi, 2003).Although our knowledge of molecules that control the morphology and functional properties of dendritic spines has expanded, information about the structures from which spines emerge is lacking. Dendritic filopodia, thin protrusions ranging in length from 2 to 35 m, are thought to participate in synaptogenesis, dendritic branching and the development of spines. During synaptogenesis, filopodia decorate the dendrites of neurons. Studies show that dendritic filopodia exhibit highly dynamic protrusive motility during periods of active synaptogenesis (Dailey and Smith, 1996;Ziv and Smith, 1996;Marrs et al., 2001). Thus, filopodia are thought to function by extending and probing the environment for appropriate presynaptic partners, thereby aiding in synapse formation. These results are further supported by electron microscopy studies which show that synapses can be formed at the tip and base of dendritic filopodia (Fiala et al., 1998;Kirov et al., 2004). As synapses form, the number of filopodia d...
Dendritic filopodia are dynamic protrusions that are thought to play an active role in synaptogenesis and serve as precursors to spine synapses. However, this hypothesis is largely based on a temporal correlation between filopodia formation and synaptogenesis. We investigated the role of filopodia in synapse formation by contrasting the roles of molecules that affect filopodia elaboration and motility, versus those that impact synapse induction and maturation. We used a filopodia inducing motif that is found in GAP-43, as a molecular tool, and found this palmitoylated motif enhanced filopodia number and motility, but reduced the probability of forming a stable axon-dendrite contact. Conversely, expression of neuroligin-1 (NLG-1), a synapse inducing cell adhesion molecule, resulted in a decrease in filopodia motility, but an increase in the number of stable axonal contacts. Moreover, RNAi knockdown of NLG-1 reduced the number of presynaptic contacts formed. Postsynaptic scaffolding proteins such as Shank1b, a protein that induces the maturation of spine synapses, increased the rate at which filopodia transformed into spines by stabilization of the initial contact with axons. Taken together, these results suggest that increased filopodia stability and not density, may be the rate-limiting step for synapse formation.
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