Actin is the major cytoskeletal source of dendritic spines, which are highly specialized protuberances on the neuronal surface where excitatory synaptic transmission occurs (Harris, K.M., and S.B. Kater. 1994. Annu. Rev. Neurosci. 17:341–371; Yuste, R., and D.W. Tank. 1996. Neuron. 16:701–716). Stimulation of excitatory synapses induces changes in spine shape via localized rearrangements of the actin cytoskeleton (Matus, A. 2000. Science. 290:754–758; Nagerl, U.V., N. Eberhorn, S.B. Cambridge, and T. Bonhoeffer. 2004. Neuron. 44:759–767). However, what remains elusive are the precise molecular mechanisms by which different neurotransmitter receptors forward information to the underlying actin cytoskeleton. We show that in cultured hippocampal neurons as well as in whole brain synaptosomal fractions, RhoA associates with glutamate receptors (GluRs) at the spine plasma membrane. Activation of ionotropic GluRs leads to the detachment of RhoA from these receptors and its recruitment to metabotropic GluRs. Concomitantly, this triggers a local reduction of RhoA activity, which, in turn, inactivates downstream kinase RhoA-specific kinase, resulting in restricted actin instability and dendritic spine collapse. These data provide a direct mechanistic link between neurotransmitter receptor activity and the changes in spine shape that are thought to play a crucial role in synaptic strength.

(1996). Modulatory role of drebrin on the cytoskeleton within dendritic spines in the rat cerebral cortex.

The actin cytoskeleton is best known for its role during cellular morphogenesis. However, other evidence suggests that actin is also crucial for the organization and dynamics of membrane organelles such as endosomes and the Golgi complex. As in morphogenesis, the Rho family of small GTPases are key mediators of organelle actin-driven events, although it is unclear how these ubiquitously distributed proteins are activated to regulate actin dynamics in an organelle-specific manner. Here we show that the brain-specific Rho-binding protein Citron-N is enriched at, and associates with, the Golgi apparatus of hippocampal neurons in culture. Suppression of the whole protein or expression of a mutant form lacking the Rho-binding activity results in dispersion of the Golgi apparatus. In contrast, high intracellular levels induce localized accumulation of RhoA and filamentous actin, protecting the Golgi from the rupture normally produced by actin depolymerization. Biochemical and functional analyses indicate that Citron-N controls actin locally by assembling together the Rho effector ROCK-II and the actin-binding, neuron-specific, protein Profilin-IIa (PIIa). Together with recent data on endosomal dynamics, our results highlight the importance of organelle-specific Rho modulators for actin-dependent organelle organization and dynamics.

Several evidences suggest that astrocytes release small transmitter molecules, peptides, and protein factors via regulated exocytosis, implying that they function as specialized neurosecretory cells. However, very little is known about the molecular and functional properties of regulated secretion in astrocytes in the adult brain. Establishing these properties is central to the understanding of the communication mode(s) of these cells and their role(s) in the control of synaptic functions and of cerebral blood flow. In this study, we have set-up a high-resolution confocal microscopy approach to distinguish protein expression in astrocytic structures and neighboring synaptic terminals in adult brain tissue. This approach was applied to investigate the expression pattern of core SNARE proteins for vesicle fusion in the dentate gyrus and CA1 regions of the mouse hippocampus. Our comparative analysis shows that astrocytes abundantly express, in their cell body and main processes, all three protein partners necessary to form an operational SNARE complex but not in the same isoforms expressed in neighbouring synaptic terminals. Thus, SNAP25 and VAMP2 are absent from astrocytic processes and typically concentrated in terminals, while SNAP23 and VAMP3 have the opposite expression pattern. Syntaxin 1 is present in both synaptic terminals and astrocytes. These data support the view that astrocytes in the adult hippocampus can communicate via regulated exocytosis and also indicates that astrocytic exocytosis may differ in its properties from action potential-dependent exocytosis at neuronal synapses, as it relies on a distinctive set of SNARE proteins.

No abstract