Paralemmin-1, a Modulator of Filopodia Induction Is Required for Spine Maturation

Abstract

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...

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