Content:
Neural development is controlled by the temporal and spatial coordination of intrinsic and extrinsic factors that may be brain region-specific. In this work, the regulation of postnatal hippocampal neuronal proliferation, differentiation and apoptosis by extrinsic factors including glucocorticoids, neurotransmitters and cytokines was investigated. Hippocampal and cerebellar granule neuronal cultures were used to identify factors which may be responsible for the differential developmental patterns displayed by granule neurons in cells from these morphologically similar brain areas, and the signaling pathways employed were also studied. Other in vitro studies were done to examine the mechanisms underlying glucocorticoid-induced apoptosis in the hippocampus, in particular with regard to the role of glutamatergic transmission. Results show that transforming growth factor β2 (TGF-β2), supported by the Smad signaling machinery, plays a key role in determining hippocampal cell fate by inhibiting proliferation and, in parallel, inducing neuronal maturation. Brain-derived neurotrophic factor (BDNF), better known for Trk receptor-mediated promotion of neurogenesis and differentiation, was found to exert anti-proliferative and pro-neuronal effects on developing hippocampal neurons by activating a MAP kinase cascade which interacted with the TGF-β signaling pathway. The glucocorticoid dexamethasone (DEX) was found to directly induce apoptosis in hippocampal cell cultures through the mediation of glucocorticoid receptors (GR); this effect could only be demonstrated if neuroprotective mineralcorticoid receptors (MR) were antagonized. Further, it was shown that mature, rather than immature, granule neurons are targeted by glucocorticoids for apoptosis. Additional experiments showed that glucocorticoid actions at least partially depend on the prevailing glutamatergic status. The apoptotic actions of DEX are, at least partly, mediated by NMDA andmetabotropic glutamate receptors. Low doses of NMDA, acting via the synaptic NMDA receptor (NMDAR) were shown to efficiently block DEX-induced hippocampal cell death. Evidence was obtained to show that glucocorticoid-induced apoptosis in hippocampal cells is mediated by NMDAR as well as metabotropic receptors. Thus, the final outcome of glucocorticoid treatment on hippocampal cell survival depends on the convergence and integration of transcriptional signals and signals originating at the cell membrane. In conclusion, these studies have identified some of the factors and signaling pathways contributing to the orchestrated neurodevelopment of hippocampal, as well as cerebellar, neurons. ; O desenvolvimento do sistema nervoso é o corolário de uma coordenação temporo-espacial de vários factores intrínsecos e extrínsecos, que apresentam, adicionalmente, importantes variações regionais. Neste trabalho dissertação foi avaliada a influência de factores extrínsecos, incluindo os glucocorticóides, neurotransmissores e citoquinas, nos mecanismos reguladores da neurogénese (proliferação, diferenciação e apoptose) pós-natal do hipocampo. Foram utilizadas culturas de neurónios hipocampais e cerebelosos para identificar factores e cascatas de sinalização responsáveis pelos padrões distintos de desenvolvimento das células granulares dessas áreas cerebrais. Foram ainda realizados estudos in vitro para analisar os mecanismos subjacentes à apoptose hipocampal induzida por glucocorticóides, em particular aqueles que envolvem a transmissão glutamatérgica. Os resultados obtidos demonstram que o transforming growth factor β2" (TGF-β2), e a vias de sinalização Smad, desempenha um papel determinante no destino das células hipocampais porque inibe a sua proliferação e, em paralelo, promove a diferenciação neuronal. Ao invés, o "brain-derived neurotrophic factor" (BDNF), conhecido por promover neurogénese e diferenciação neuronal através de receptores tirosina cínase (Trk-r), revelou um efeito anti-proliferativo e favorecedor da maturação neuronal pela activação da cascata da cínase MAP que, por seu turno, interactua com a via de sinalização do TGF-β. Demontrou-se ainda que a dexametasona, um glucocorticóide, induzia directamente apoptose em culturas de células hipocampais, numa acção mediada pelos receptores dos glucocorticóides (GR)
Note:
Dissertation 2004
Language:
English
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