title: Neuronal compartments and axonal transport of synapsin I.
abstract: Studies on the transport kinetics and the posttranslational modification of synapsin I in mouse retinal ganglion cells were performed to obtain an insight into the possible factors involved in forming the structural and functional differences between the axon and its terminals. Synapsin I, a neuronal phosphoprotein associated with small synaptic vesicles and cytoskeletal elements at the presynaptic terminals, is thought to be involved in modulating neurotransmitter release. The state of phosphorylation of synapsin I in vitro regulates its interaction with both synaptic vesicles and cytoskeletal components, including microtubules and microfilaments. Here we present the first evidence that in the mouse retinal ganglion cells most synapsin I is transported down the axon, together with the cytomatrix proteins, at the same rate as the slow component b of axonal transport, and is phosphorylated at both the head and tail regions. In addition, our data suggest that, after synapsin I has reached the nerve endings, the relative proportions of variously phosphorylated synapsin I molecules change, and that these changes lead to a decrease in the overall content of phosphorus. These results are consistent with the hypothesis that, in vivo, the phosphorylation of synapsin I along the axon prevents the formation of a dense network that could impair organelle movement. On the other hand, the dephosphorylation of synapsin I at the nerve endings may regulate the clustering of small synaptic vesicles and modulate neurotransmitter release by controlling the availability of small synaptic vesicles for exocytosis.
mesh_terms: D000818:Animals; D001370:Axonal Transport; D007700:Kinetics; D008297:Male; D008715:Methionine / Q000378:metabolism; D051379:Mice; D008810:Mice, Inbred C57BL; D009411:Nerve Endings / Q000502:physiology; D009474:Neurons / Q000378:metabolism / Q000502:physiology*; D009900:Optic Nerve / Q000502:physiology*; D010710:Phosphates / Q000378:metabolism; D010761:Phosphorus Radioisotopes; D010766:Phosphorylation; D012165:Retinal Ganglion Cells / Q000378:metabolism / Q000502:physiology*; D013462:Sulfur Radioisotopes; D013477:Superior Colliculi / Q000502:physiology*; D016704:Synapsins / Q000096:biosynthesis / Q000302:isolation & purification / Q000378:metabolism*; D014795:Visual Pathways / Q000502:physiology*
chemical_list: D010710:Phosphates; D010761:Phosphorus Radioisotopes; D013462:Sulfur Radioisotopes; D016704:Synapsins; D008715:Methionine