Semaphorin 4D/Plexin-B1-Mediated M-Ras GAP Activity Regulates Actin-Based Dendrite Remodeling through Lamellipodin

Abstract

Semaphorins have been identified as repulsive guidance molecules in the developing nervous system. We recently reported that the semaphorin 4D (Sema4D) receptor Plexin-B1 induces repulsion in axon and dendrites by functioning as a GTPase-activating protein (GAP) for R-Ras and M-Ras, respectively. In axons, Sema4D stimulation induces growth cone collapse, and downregulation of R-Ras activity by Plexin-B1-mediated GAP activity is required for the action. Axonal R-Ras GAP activity downregulates phosphatidylinositol 3-kinase signaling pathway, and thereby induces inactivation of a microtubule assembly promoter protein, CRMP-2. However, in contrast to the well studied roles of semaphorins and plexins in axonal guidance, signaling molecules linking M-Ras GAP to dendritic cytoskeleton remain obscure. Here we identified an Ena/VASP ligand, Lamellipodin (Lpd), as a novel effector of M-Ras in dendrites. Lpd was expressed in F-actin-rich distal dendritic processes and was required for both basal and M-Ras-mediated dendrite development. Subcellular fractionation showed M-Ras-dependent membrane translocation of Lpd, which was suppressed by Sema4D. Furthermore, the Ena/VASP-binding region within Lpd was required for dendrite development, and its membrane targeting was sufficient to overcome the Sema4D-mediated reduction of dendritic outgrowth and disappearance of F-actin from distal dendrites. Furthermore, in utero electroporation experiments also indicated that regulation of the M-Ras-Lpd system by the GAP activity of Plexin is involved in the normal development of cortical dendrites in vivo. Overall, our study sheds light on how repulsive guidance molecules regulate actin cytoskeleton in dendrites, revealing a novel mechanism that the M-Ras-Lpd system regulates actin-based dendrite remodeling by Sema/Plexin in rats or mice of either sex.

Structural and Mechanistic Insights into the Recruitment of Talin by RIAM in Integrin Signaling

“…Lpd has been identified as an M-Ras effector protein but retains a moderate Rap1-binding affinity owing to an RA-PH functional module highly similar to that of RIAM (Lafuente et al, 2004; Tasaka et al, 2012; Yoshikawa et al, 2007; Zhang et al, 2014). Lpd also possesses all R8-interacting residues in RIAM TBS1 in its N-terminal talin-binding site (Lpd-TBS) except a single substitution of Leu15 RIAM with Trp31 Lpd (Fig.…”

Phosphatidylinositol (3,4) bisphosphate-specific phosphatases and effector proteins: A distinct branch of PI3K signaling

“…While direct binding of the Lpd RA domain to Ras (K-Ras) was not detected in yeast two-hybrid or in vitro binding experiments [17], pull-down assays suggested that Lpd can associate with activated K-Ras, NRas, H-Ras, and R-Ras [82] as well as M-Ras and is required for M-Ras-mediated neuronal 15 dendrite remodeling [83]. The PH domain of Lpd was found to specifically bind to PI(3,4)P2, but not other phosphoinositides (see discussion below).…”

Are Molecules Involved in Neuritogenesis and Axon Guidance Related to Autism Pathogenesis?

“…In growth cones, EnaH/VASP proteins are concentrated at filopodia tips (Lebrand et al 2004). EnaH/VASP complex is also required for dendrite growth (Tasaka et al 2012). EnaH/ VASP proteins at the tips of lamellipodia and filopodia accelerate actin polymerization by their anti-capping and bundling activity , and neurons lacking these proteins fail to form neurites (Kwiatkowski et al 2007).…”