Biomedical subjects

Jorge Santos Da Silva

Publications and source records attributed to Jorge Santos Da Silva.

6 recordsLinked to original sources

PubMed2006

Localized recruitment and activation of RhoA underlies dendritic spine morphology in a glutamate receptor-dependent manner.

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.

6-Cyano-7-nitroquinoxaline-2,3-dione↗

PubMed2005

Centrosome localization determines neuronal polarity.

Neuronal polarization occurs shortly after mitosis. In neurons differentiating in vitro, axon formation follows the segregation of growth-promoting activities to only one of the multiple neurites that form after mitosis. It is unresolved whether such spatial restriction makes use of an intrinsic program, like during C. elegans embryo polarization, or is extrinsic and cue-mediated, as in migratory cells. Here we show that in hippocampal neurons in vitro, the axon consistently arises from the neurite that develops first after mitosis. Centrosomes, the Golgi apparatus and endosomes cluster together close to the area where the first neurite will form, which is in turn opposite from the plane of the last mitotic division. We show that the polarized activities of these organelles are necessary and sufficient for neuronal polarization: (1) polarized microtubule polymerization and membrane transport precedes first neurite formation, (2) neurons with more than one centrosome sprout more than one axon and (3) suppression of centrosome-mediated functions precludes polarization. We conclude that asymmetric centrosome-mediated dynamics in the early post-mitotic stage instruct neuronal polarity, implying that pre-mitotic mechanisms with a role in division orientation may in turn participate in this event.

Animals↗

PubMed2005

Asymmetric membrane ganglioside sialidase activity specifies axonal fate.

Axon specification triggers the polarization of neurons and requires the localized destabilization of filamentous actin. Here we show that plasma membrane ganglioside sialidase (PMGS) asymmetrically accumulates at the tip of one neurite of the unpolarized rat neuron, inducing actin instability. Suppressing PMGS activity blocks axonal generation, whereas stimulating it accelerates the formation of a single (not several) axon. PMGS induces axon specification by enhancing TrkA activity locally, which triggers phosphatidylinositol-3-kinase (PI3K)- and Rac1-dependent inhibition of RhoA signaling and the consequent actin depolymerization in one neurite only. Thus, spatial restriction of an actin-regulating molecular machinery, in this case a membrane enzymatic activity, before polarization is enough to determine axonal fate.

Actins↗

PubMed2004

RhoA, Rac1, and cdc42 intracellular distribution shift during hippocampal neuron development.

Differences in cytoskeleton organization are key determinants of the architecture and dynamics of axons and dendrites. This is most clearly illustrated by the diverse pools of microtubule-associated proteins in axons and dendrites. Whether similar polarized organization occurs for actin regulatory proteins remains to be determined. To address this issue, we analyzed the intracellular distribution of the Rho GTPases, RhoA, Rac1, and cdc42 in hippocampal neurons in culture. We report that all three Rho members are evenly distributed during the time of axon and dendrite sprouting. This is not the case in mature neurons, as RhoA enriches in dendrites, Rac1 in axons, and Cdc42 is equally abundant in both domains. Polarized segregation of the actin regulatory machinery in mature neurons might play an important role in axonal and dendritic architectural plasticity.

Actin Cytoskeleton↗

PubMed2003

Proteomic characterisation of neuronal sphingolipid-cholesterol microdomains: role in plasminogen activation.

Sorting of certain membrane proteins requires a mechanism involving rafts, protein-lipid complexes enriched in glycosphingolipids and cholesterol. These microdomains remain at the plasma membrane of different cell types and play a role in signal transduction. Although recent reports have begun to describe molecules associated with rafts, their protein composition remains largely unknown, especially in neuronal cells. To address this question, we have purified detergent-insoluble raft fractions (DRMs) from primary cultures of hippocampal neurons. Bidimensional gel analysis and pharmacological raft lipid manipulation allowed the identification of neuronal raft proteins and their characterisation by MALDI-TOF analysis. Enolases were found among the proteins identified and functional studies demonstrate their participation in plasminogen binding. We also show the specific enrichment in rafts of several other plasminogen binding molecules and the exclusive activation of plasminogen to the protease plasmin in these microdomains. These observations suggest that neuronal rafts may play, in addition to intracellular signaling, a role in extracellular/membrane protein proteolysis.

Animals↗

PubMed2003

RhoA/ROCK regulation of neuritogenesis via profilin IIa-mediated control of actin stability.

Neuritogenesis, the first step of neuronal differentiation, takes place as nascent neurites bud from the immediate postmitotic neuronal soma. Little is known about the mechanisms underlying the dramatic morphological changes that characterize this event. Here, we show that RhoA activity plays a decisive role during neuritogenesis of cultured hippocampal neurons by recruiting and activating its specific kinase ROCK, which, in turn, complexes with profilin IIa. We establish that this previously uncharacterized brain-specific actin-binding protein controls neurite sprouting by modifying actin stability, a function regulated by ROCK-mediated phosphorylation. Furthermore, we determine that this novel cascade is switched on or off by physiological stimuli. We propose that RhoA/ROCK/PIIa-mediated regulation of actin stability, shown to be essential for neuritogenesis, may constitute a central mechanism throughout neuronal differentiation.

Actin Cytoskeleton↗