Synapsin I (Protein I), a Nerve Terminal-Specific Phosphoprotein. I. Its General Distribution in Synapses of The Central and Peripheral Nervous System Demonstrated by Immunofluorescence in Frozen and Plastic Sections

Synapsin I (formerly referred to as protein I) is the collective name for two almost identical phosphoproteins, synapsin la and synapsin Ib (protein la and protein Ib), present in the nervous system. Synapsin I has previously been shown by immunoperoxidase studies (De Camilli, P., T. Ueda, F. E. Bloom, E. Battenberg, and P. Greengard, 1979, Proc. Natl. Acad. ScL USA, 76:5977-5981; Bloom, F. E., T. Ueda, E. Battenberg, and P. Greengard, 1979, Proc. Natl. Acad. 5ci. USA 76:5982-5986) to be a neuron-specific protein, present in both the central and peripheral nervous systems and concentrated in the synaptic region of nerve cells. In those preliminary studies, the occurrence of synapsin I could be demonstrated in only a portion of synapses. We have now carried out a detailed examination of the distribution of synapsin I immunoreactivity in the central and peripheral nervous systems. In this study we have attempted to maximize the level of resolution of immunohistochemical light microscopy images in order to estimate the proportion of immunoreactive synapses and to establish their precise distribution. Optimal results were obtained by the use of immunofluorescence in semithin sections (~1 #m) prepared from Epon-embedded nonosmicated tissues after the Epon had been removed. Our results confirm the previous observations on the specific localization of synapsin I in nerve cells and synapses. In addition, the results strongly suggest that, with a few possible exceptions involving highly specialized neurons, all synapses contain synapsin I. Finally, immunocytochemical experiments indicate that synapsin I appearance in the various regions of the developing nervous system correlates topographically and temporally with the appearance of synapses. In two accompanying papers (De Camilli, P., S. M. Harris, Jr., W. B. Huttner, and P. Greengard, and Huttner, W. B., W. Schiebler, P. Greengard, and P. De Camilli, 1983, J. Cell Biol. 96:13551373 and 1374-1388, respectively), evidence is presented that synapsin I is specifically associated with synaptic vesicles in nerve endings. Synapsin I (formerly referred to as protein I) is the collective name for two peptides, synapsin Ia and synapsin Ib (protein Ia and protein Ib) with very similar properties. It was discovered as a major endogenous substrate for cAMP-dependent phosphorylation in mammalian brain (26, 50). Later studies showed that synapsin I is also a major endogenous substrate for Ca/ calmodulin-dependent phosphorylation in brain (22, 23, 27, 28, 46). Synapsin I has been purified to homogeneity from THE JOURNAL OF CELL BIOLOGY VOLUME 96 MAY 1983 1337-1354 © The Rockefeller University Press • 0021-9525/83/05/1337/18 $1.0