Spatially patterned gradients of synaptic connectivity are established early in the developing retina

Retinal neurons receive input from other cells via synapses and the position of these synapses on the neurons reflects the retinal regions from which information is received. A new study in Neural Development establishes that the spatial distribution of excitatory synaptic inputs emerges at the onset of synapse formation rather than as a result of changes during neuronal reorganisation

Cell-Autonomous Alterations in Dendritic Arbor Morphology and Connectivity Induced by Overexpression of MeCP2 in Xenopus Central Neurons In Vivo

Methyl CpG binding protein-2 (MeCP2) is an essential epigenetic regulator in human brain development. Mutations in the MeCP2 gene have been linked to Rett syndrome, a severe X-linked progressive neurodevelopmental disorder, and one of the most common causes of mental retardation in females. MeCP2 duplication and triplication have also been found to affect brain development, indicating that both loss of function and gain in MeCP2 dosage lead to similar neurological phenotypes. Here, we used the Xenopus laevis visual system as an in vivo model to examine the consequence of increased MeCP2 expression during the morphological maturation of individual central neurons in an otherwise intact brain. Single-cell overexpression of wild-type human MeCP2 was combined with time-lapse confocal microscopy imaging to study dynamic mechanisms by which MeCP2 influences tectal neuron dendritic arborization. Analysis of neurons co-expressing DsRed2 demonstrates that MeCP2 overexpression specifically interfered with dendritic elaboration, decreasing the rates of branch addition and elimination over a 48 hour observation period. Moreover, dynamic analysis of neurons co-expressing wt-hMeCP2 and PSD95-GFP revealed that even though neurons expressing wt-hMeCP2 possessed significantly fewer dendrites and simpler morphologies than control neurons at the same developmental stage, postsynaptic site density in wt-hMeCP2-expressing neurons was similar to controls and increased at a rate higher than controls. Together, our in vivo studies support an early, cell-autonomous role for MeCP2 during the morphological differentiation of neurons and indicate that perturbations in MeCP2 gene dosage result in deficits in dendritic arborization that can be compensated, at least in part, by synaptic connectivity changes

Regulation of Cerebellar Purkinje Cell Development: Interactions Between Trophic Factors and Neurotransmitters

Previous study indicates that in the cerebellum. Nerve Growth Factor (NGF) and NGF receptors are highly expressed during late embryonic and early postnatal development, suggesting that NGF may play an important role in cerebellar ontogeny. However, while receptor had been localized to some cerebellar populations, delineation of receptor subtypes, and potential physiologic function remained to the defined. The objective of this thesis was to define the role that NGF plays in cerebellar ontogeny. Initial studies were devoted to the localization of NGF binding sites to cell types in the developing cerebellum, and to the definition of receptor subtypes. 125I-.NGF binding was employed to delineate low- and high-affinity receptor sites in the postnatal rat cerebellum. Both high-affinity and low-affinity receptor sites were localized to cell bodies, dendrites and axonal processes of Purkinje cells. The expression of the biologically active, high-affinity receptors was found to correlate with cerebellar development, and suggested that N G F may regulate the normal ontogeny of Purkinje cells. The development of cerebellar Purkinje cells is subject to regulation by multiple epigenetic signals. To define mechanisms by which trophic and presynaptic stimulation may potentially regulate Purkinje cell ontogeny, I studied the effects of N G F and excitatory transmitters on Purkinje cell survival and morphological maturation in dissociated cell culture. Purkinje cells were identified by expression of Vitamin D-dependent Calcium Binding Protein (CaBP), and by their characteristic morphology. N G F receptors were selectively localized to Purkinje cells both by ligand and monoclonal antibody binding, suggesting responsivity to the trophic agent. Simultaneous exposure to the pharmacologic depolarizing agents, high-potassium or veratridine, and N G F specifically enhanced Purkinje cell survival in culture. Furthermore, N G F together with the excitatory neurotransmitters, aspartate or glutamate, promoted a 2-fold increase in survival. In addition, N G F increased Purkinje cell size and promoted neurite elaboration. These effects required simultaneous exposure to N G F and either aspartate, glutamate or pharmacologic depolarizing agents. Effects on survival or neurite elaboration were not evoked by exposure to trophic factor or transmitters alone. These results suggested a novel mechanism for regulation of development, in which trophic factors and afferent stimulation interact. To begin to investigate potential mechanisms by which excitatory signals and NGF interact in the regulation of Purkinje cell development I studied regulation of NGF receptor expression by excitatory influences in dissociated cerebellar cell cultures. Sensitive receptor-protein cross-linking, and ribonuclease protection assays were employed to evaluate NGF receptor protein, and message levels respectively. Exposure of neurons in culture to either pharmacologic depolarizing agents, or to the normal afferent neurotransmitter aspartate, resulted in a 2 to 3 fold increase in N G F receptor expression. In addition to the N G F receptor, expression of the NGF gene was examined. Cerebellar glia were identified as the source of N G F expression, and the synthesis of the trophic factor was also subject to regulation by depolarizing signals. Together, the results presented in this thesis suggest that afferent innervation and N G F interact to promote survival and morphogenesis of developing Purkinje cells