The F-BAR Protein Rapostlin Regulates Dendritic Spine Formation in Hippocampal Neurons

Abstract

Pombe Cdc15 homology proteins, characterized by Fer/CIP4 homology Bin-Amphiphysin-Rvs/extended Fer/CIP4 homology (F-BAR/EFC) domains with membrane invaginating property, play critical roles in a variety of membrane reorganization processes. Among them, Rapostlin/formin-binding protein 17 (FBP17) has attracted increasing attention as a critical coordinator of endocytosis. Here we found that Rapostlin was expressed in the developing rat brain, including the hippocampus, in late developmental stages when accelerated dendritic spine formation and maturation occur. In primary cultured rat hippocampal neurons, knockdown of Rapostlin by shRNA or overexpression of Rapostlin-QQ, an F-BAR domain mutant of Rapostlin that has no ability to induce membrane invagination, led to a significant decrease in spine density. Expression of shRNA-resistant wild-type Rapostlin effectively restored spine density in Rapostlin knockdown neurons, whereas expression of Rapostlin deletion mutants lacking the protein kinase C-related kinase homology region 1 (HR1) or Src homology 3 (SH3) domain did not. In addition, knockdown of Rapostlin or overexpression of Rapostlin-QQ reduced the uptake of transferrin in hippocampal neurons. Knockdown of Rnd2, which binds to the HR1 domain of Rapostlin, also reduced spine density and the transferrin uptake. These results suggest that Rapostlin and Rnd2 cooperatively regulate spine density. Indeed, Rnd2 enhanced the Rapostlin-induced tubular membrane invagination. We conclude that the F-BAR protein Rapostlin, whose activity is regulated by Rnd2, plays a key role in spine formation through the regulation of membrane dynamics.

Different functional modes of BAR domain proteins in formation and plasticity of mammalian postsynapses

“…In line with the data for BAR domain proteins in nonneuronal cells , endophilin (Chowdhury et al, 2006), PICK1 (Xia et al, 1999(Xia et al, , 2000, rapostlin (Wakita et al, 2011) and syndapin I (Pérez-Otaño et al, 2006;Anggono et al, 2013) have been implicated in the endocytic uptake of different postsynaptic receptors, such as transferrin, AMPA or N-methyl-Daspartate (NMDA) receptors. However, to our knowledge, only in the case of PICK1 has BAR-domain-mediated lipid binding explicitly been demonstrated to be required for regulating neurotransmitter receptor surface expression and clustering (Jin et al, 2006).…”

“…2). For instance, knockdown of FBP17 leads to reduced spine densities (Wakita et al, 2011). Similarly, knockout of syndapin I leads to a reduction of dendritic spine density, which we have shown is caused by postsynaptic loss of syndapin I (Schneider et al, 2014) and does not result from neurotransmission defects that are caused by the presynaptic membrane trafficking impairments that are also seen upon syndapin I knockout .…”

Transcription factor Sp4 regulates expression of nervous wreck 2 to control NMDAR1 levels and dendrite patterning

“…). Several other F‐BAR family proteins have also been shown to regulate neurite morphogenesis including CIP4, Pacsin1/syndapin1, Raspotlin, and WRP/srGAP3, a protein implicated in mental retardation (Dharmalingam et al, ; Guerrier et al, ; Carlson et al, ; Wakita et al, ; Saengsawang et al, ). Further, our data suggest that Nwk2 regulates dendrite patterning, at least in part, through promoting surface levels of NR1.…”

Formins and membranes: anchoring cortical actin to the cell wall and beyond

“…Yeast and mammalian formin interactors such as FBP1/FBP17/Rapostlin (Wakita et al, 2011), FNBP1L/Toca (Huett et al, 2009) or CIP4 (Aspenström et al, 2006) all share a common architecture with an N-terminal F-BAR domain and C-terminal SH3 domain, with a coiled coil motif in between (Roberts-Galbraith and Gould, 2010). A mammalian homolog of CIP4, a prototype protein of this family originally identified as a Cdc42 (RHO GTPase) effector, interacts with the DAAM1 formin via its SH3 domain, raising thus the possibility that other SH3-containing proteins may bind formins as well (Aspenström et al, 2006).…”

“…Phosphoinositide interaction of Drosophila Diaphanous is required for targeting the formin to the epithelial apical membrane (Rousso et al, 2013), and interaction with the F-BAR protein CIP4 may inhibit Diaphanous in lateral and basal membrane domains (Yan et al, 2013). However, other metazoan F-BAR proteins may stimulate formin activity while connecting the plasmalemma and the cortical cytoskeleton during actin-driven membrane tubulation and ruffling (Toguchi et al, 2010) or during formation of dendritic spines in neurons (Wakita et al, 2011). Aspergillus formin interactor MesA promotes formin localization to growing tips of hyphae (Pearson et al, 2004), reminiscent of the function of some plant formins in tip growth (see below).…”