Browsing by Subject "Axon hillock"

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Der Beitrag von Neurofascin zur Entstehung und Lokalisation von Gephyrinclustern während der inhibitorischen Synaptogenese im ZNS
(2008) Burkarth, Nadine; Volkmer, Hansjürgen
Membrane-bound cell adhesion molecules (CAMs) were discovered to be key players in initiating excitatory synapse formation in the CNS. However, so far little is known about the role of CAMs in inhibitory synapse development. In particular, a functional link between CAMs and the clustering of postsynaptic scaffold component gephyrin, which is a critical determinant of y-aminobutyric acid A (GABAA) clustering, still needs to be elaborated. Neurofascin belongs to the L1-subgroup of the immunoglobuline like CAMs (IgCAMs). In vivo Neurofascin has been shown to direct the formation and localization of GABAergic Input on cerebellar Purkinje neurons. Thus it serves as a candidate molecule recruiting gephyrin to inhibitory postsynaptic sites. At early stages of inhibitory synaptogenesis, formation of gephyrin clusters and their translocation to the axon hillock correlated with a somatic expression of neurofascin in rat hippocampal neurons. Furthermore, a neurofascin splice variant lacking the extracellular fifth fibronectine-III like domain (NF-5te FN-III) was predominantely expressed at early stages of synapse formation. In contrast expression of NF+5te FN-III harbouring the fifth fibronectine-III like domain was prominent only in adult brain. Transfection of expression vectors for different splice variants and deletion mutants of neurofascin revealed that the embryonic neurofascin isoform NF-5te FN-III is required for the formation of gephyrin clusters. This process is presumably dependent on extracellular interactions with molecules on pre- and postsynaptic terminals, respectively. However, possible interaction partners for neurofascin are unknown. Point mutations in the cytoplasmatic domain of neurofascin inhibited the formation of gephyrin clusters suggesting intracellular signal transduction pathways triggered by neurofascin. Furthermore, expression of neurofascin is necessary for the translocation of gephyrin clusters to the axon hillock of hippocampal neurons as shown by shRNA-mediated knockdown. In addition, overexpression of an embryonic neurofascin isoform was sufficient for functional rescue after knockdown of endogenous neurofascin. Expression of NF-5te FN-III resulted in the accumulation of exogenous GFP-Gephyrin at the axon initial segment AIS suggesting a functional link between neurofascin and gephyrin. However, colocalization studies in HEK293 and PC12E2 cells, respectively, did not provide an indication of direct neurofascin gephyrin interactions leading to the observed effects.