No abstract

Dendritic spines are the primary postsynaptic sites of excitatory neurotransmission in the brain. They exhibit a remarkable morphological variety, ranging from thin protrusions, to stubby shapes, to bulbous mushroom shapes. The remodeling of spines is thought to regulate the strength of the synaptic connection, which depends vitally on the number and the spatial distribution of AMPA-type glutamate receptors (AMPARs). We present numerical and analytical analyses demonstrating that this shape strongly affects AMPAR diffusion. We report a pronounced suppression of the receptor exit rate out of spines with decreasing neck radius. Thus, mushroomlike spines become highly effective at retaining receptors in the spine head. Moreover, we show that the postsynaptic density further enhances receptor trapping, particularly in mushroomlike spines local exocytosis in the spine head, in contrast to release at the base, provides rapid and specific regulatory control of AMPAR concentration at synapses.

Cell membrane deformations are crucial for proper cell function. Specialized protein assemblies initiate inward or outward membrane deformations that the cell uses respectively to uptake external substances or probe the environment. The assembly and dynamics of the actin cytoskeleton are involved in this process, although their detailed role remains controversial. We show here that a dynamic, branched actin network is sufficient to initiate both inward and outward membrane deformation. The polymerization of a dense actin network at the membrane of liposomes produces inward membrane bending at low tension, while outward deformations are robustly generated regardless of tension. Our results shed light on the mechanism cells use to internalize material, both in mammalian cells, where actin polymerization forces are required when membrane tension is increased, and in yeast, where those forces are necessary to overcome the opposing turgor pressure. By combining experimental observations with physical modeling, we propose a mechanism that explains how membrane tension and the architecture of the actin network regulate cell-like membrane deformations.How the same branched actin structure can be responsible for the initiation of filopodia, which are outward-pointing membrane deformations, as well as endocytic invaginations that deform the membrane inward, is what we want to address in this paper. Such a question is difficult to investigate in cells that contain redundant mechanisms for cell deformation. Actin dynamics triggered at a liposome membrane provide a control on experimental parameters such as membrane composition, curvature and tension, and allow the specific role of actin dynamics to be addressed. We unambiguously show that the same branched actin network is able to produce both endocytosis-like and dendritic filopodia-like deformations. With a theoretical model, we predict under which conditions the stress exerted on the membrane will lead to inward and/or outward pointing membrane deformations. Combining experiments and theory allows us to decipher how the interplay between membrane tension, actin dynamics, and actin network structure produces inward or outward membrane deformations. Membrane deformations: tubes and spikesLiposomes are covered with an activator of the Arp2/3 complex, pVCA, the proline rich domain-verprolin homology-central-acidic sequence from human WASP, which is purified with a streptavidin tag, and that we call hereafter S-pVCA. A branched actin network grows at their surface when placed in a mixture containing monomeric actin, profilin, the Arp2/3 complex and capping protein (CP) ("reference condition", Methods and Fig. 1a). Strikingly, the membrane of liposomes is not smooth, but instead displays a rugged profile: membrane tubes, hereafter called "tubes", radiate from the liposome surface and extend into the actin network (Fig. 1b), even when comet formation has occurred 7, 8 (Supplementary Fig. 1a). The initiation of these tubes is reminiscent of early stage of endocytosis. Interesting...