Impairment of axonal development and of synaptogenesis in hippocampal neurons of synapsin I-deficient mice

Differential synaptic vesicle protein expression in the barrel field of developing cortex

The distribution of four proteins associated with synaptic vesicles, SV2, synaptophysin, synapsin I, and rab3a, was investigated during postnatal development of the posteromedial barrel subfield (PMBSF) in the rat somatosensory cortex. A distinct progression in the appearance of the different synaptic vesicle proteins within the PMBSF was observed. SV2, synapsin I, and synaptophysin revealed the organization of the barrel field in the neonate. This early demarcation of the cortical representation of the vibrissal array coincides with the earliest known age for the emergence of the cytoarchitectonic organization of this region. In contrast, rab3a did not delimit the barrels until the end of the 1st postnatal week, coincident with the known onset of adult-like physiological activity and the loss of plasticity in afferents to this region. In addition, the appearance of the different synaptic vesicle proteins occurred earlier within the PMBSF than in the adjacent extra-barrel regions of the cortex. These results show that the molecular differentiation of synaptic fields across the cortex is not a homogeneous and synchronous process in terms of synaptic vesicle protein expression. Because these proteins act together in mature synapses to ensure the regulated release of neurotransmitters, our results suggest that this temporo-spatial asynchrony may underlie different potentials for synaptic activity and thus contribute to the development of cortical maps.

Biochemical and functional characterization of the synaptic vesicle-associated form of CA2+/calmodulin-dependent protein kinase II

Ca+/calmodulin-dependent protein kinase II (CaMPKII) is a brain-enriched protein kinase that plays important roles in synaptic transmission and plasticity. In nerve terminals, a form of CaMPKII is associated with synaptic vesicles and binds the COOH-terminal region of synapsin I (SYNI). The biochemical properties of the vesicle-associated form of CAMPKII have been investigated and compared with those of the soluble forebrain enzyme. Both the alpha- and beta-subunits of CaMPKII copurifying with synaptic vesicles were tightly associated with the vesicle membrane. The vesicle-associated form of CaMPKII was indistinguishable from the soluble form with respect to sites of autophosphorylation, kinetics of both autophosphorylation and SYNI phosphorylation, and induction of autonomous activity upon autophosphorylation. Although both subunits of the soluble CaMPKII interacted with a photoactivatable SYNI derivative, only the alpha-subunit of the synaptic vesicle-associated CaMPKII bound to the COOH-terminal region of SYNI. The latter interaction was strongly dependent on the phosphorylation state of SYNI and on divalent cations, but appeared to be independent of autophosphorylation. These results demonstrate that, although the vesicle-associated form of CaMPKII is catalytically indistinguishable from the soluble form, it exhibits distinct characteristics concerning its association with the vesicle membrane and with SYNI.

Synaptic vesicle recycling in synapsin I knock-out mice

The synapsins are a family of four neuron-specific phosphoproteins that have been implicated in the regulation of neurotransmitter release. Nevertheless, knock-out mice lacking synapsin Ia and Ib, family members that are major substrates for cAMP and Ca2+/ Calmodulin (CaM)-dependent protein kinases, show limited phenotypic changes when analyzed electrophysiologically (Rosahl, T.W., D. Spillane, M. Missler, J. Herz, D.K. Selig, J.R. Wolff, R.E. Hammer, R.C. Malenka, and T.C. Sudhof. 1995. Nature (Lond.). 375: 488-493; Rosahl, T.W., M. Geppert, D. Spillane, D., J. Herz, R.E. Hammer, R.C. Malenka, and T.C. Sudhof. 1993. Cell. 75:661-670; Li, L., L.S. Chin, O. Shupliakov, L. Brodin, T.S. Sihra, O. Hvalby, V. Jensen, D. Zheng, J.O. McNamara, P. Greengard, and P. Andersen. 1995. Proc. Natl. Acad. Sci. USA. 92:9235-9239; see also Pieribone, V.A., O. Shupliakov, L. Brodin, S. Hilfiker-Rothenfluh, A.J. Czernik, and P. Greengard. 1995. Nature (Lond.). 375:493-497). Here, using the optical tracer FM 1-43, we characterize the details of synaptic vesicle recycling at individual synaptic boutons in hippocampal cell cultures derived from mice lacking synapsin I or wild-type equivalents. These studies show that both the number of vesicles exocytosed during brief action potential trains and the total recycling vesicle pool are significantly reduced in the synapsin I-deficient mice, while the kinetics of endocytosis and synaptic vesicle repriming appear normal.

Invertebrate synapsins: a single gene codes for several isoforms in Drosophila

Vertebrate synapsins constitute a family of synaptic proteins that participate in the regulation of neurotransmitter release. Information on the presence of synapsin homologs in invertebrates has been inconclusive. We have now cloned a Drosophila gene coding for at least two inferred proteins that both contain a region with 50% amino acid identity to the highly conserved vesicle- and actin-binding "C" domain of vertebrate synapsins. Within the C domain coding sequence, the positions of two introns have been conserved exactly from fly to human. The positions of three additional introns within this domain are similar. The Drosophila synapsin gene (Syn) is widely expressed in the nervous system of the fly. The gene products are detected in all or nearly all conventional synaptic terminals. A single amber (UAG) stop codon terminates the open reading frame (ORF1) of the most abundant transcript of the Syn gene 140 amino acid codons downstream of the homology domain. Unexpectedly, the stop codon is followed by another 443 in-frame amino acid codons (ORF2). Using different antibodies directed against ORF1 or ORF2, we demonstrate that in the adult fly small and large synapsin isoforms are generated. The small isoforms are only recognized by antibodies against ORF1; the large isoforms bind both kinds of antibodies. We suggest that the large synapsin isoform in Drosophila may be generated by UAG read-through. Implications of such an unconventional mechanism for the generation of protein diversity from a single gene are discussed.

Invertebrate synapsins: a single gene codes for several isoforms in Drosophila

Vertebrate synapsins constitute a family of synaptic proteins that participate in the regulation of neurotransmitter release. Information on the presence of synapsin homologs in invertebrates has been inconclusive. We have now cloned a Drosophila gene coding for at least two inferred proteins that both contain a region with 50% amino acid identity to the highly conserved vesicle- and actin-binding "C" domain of vertebrate synapsins. Within the C domain coding sequence, the positions of two introns have been conserved exactly from fly to human. The positions of three additional introns within this domain are similar. The Drosophila synapsin gene (Syn) is widely expressed in the nervous system of the fly. The gene products are detected in all or nearly all conventional synaptic terminals. A single amber (UAG) stop codon terminates the open reading frame (ORF1) of the most abundant transcript of the Syn gene 140 amino acid codons downstream of the homology domain. Unexpectedly, the stop codon is followed by another 443 in-frame amino acid codons (ORF2). Using different antibodies directed against ORF1 or ORF2, we demonstrate that in the adult fly small and large synapsin isoforms are generated. The small isoforms are only recognized by antibodies against ORF1; the large isoforms bind both kinds of antibodies. We suggest that the large synapsin isoform in Drosophila may be generated by UAG read-through. Implications of such an unconventional mechanism for the generation of protein diversity from a single gene are discussed.

Neurotrophins stimulate phosphorylation of synapsin I by MAP kinase and regulate synapsin I-actin interactions

The ability of neurotrophins to modulate the survival and differentiation of neuronal populations involves the Trk/MAP (mitogen-activated protein kinase) kinase signaling pathway. More recently, neurotrophins have also been shown to regulate synaptic transmission. The synapsins are a family of neuron-specific phosphoproteins that play a role in regulation of neurotransmitter release, in axonal elongation, and in formation and maintenance of synaptic contacts. We report here that synapsin I is a downstream effector for the neurotrophin/Trk/MAP kinase cascade. Using purified components, we show that MAP kinase stoichiometrically phosphorylated synapsin I at three sites (Ser-62, Ser-67, and Ser-549). Phosphorylation of these sites was detected in rat brain homogenates, in cultured cerebrocortical neurons, and in isolated presynaptic terminals. Brain-derived neurotrophic factor and nerve growth factor upregulated phosphorylation of synapsin I at MAP kinase-dependent sites in intact cerebrocortical neurons and PC12 cells, respectively, while KCl- induced depolarization of cultured neurons decreased the phosphorylation state at these sites. MAP kinase-dependent phosphorylation of synapsin I significantly reduced its ability to promote G-actin polymerization and to bundle actin filaments. The results suggest that MAP kinase-dependent phosphorylation of synapsin I may contribute to the modulation of synaptic plasticity by neurotrophins and by other signaling pathways that converge at the level of MAP kinase activation.

Impairment of synaptic vesicle clustering and of synaptic transmission, and increased seizure propensity, in synapsin I-deficient mice

Synapsin I has been proposed to be involved in the modulation of neurotransmitter release by controlling the availability of synaptic vesicles for exocytosis. To further understand the role of synapsin I in the function of adult nerve terminals, we studied synapsin I-deficient mice generated by homologous recombination. The organization of synaptic vesicles at presynaptic terminals of synapsin I-deficient mice was markedly altered: densely packed vesicles were only present in a narrow rim at active zones, whereas the majority of vesicles were dispersed throughout the terminal area. This was in contrast to the organized vesicle clusters present in terminals of wild-type animals. Release of glutamate from nerve endings, induced by K+,4-aminopyridine, or a Ca2+ ionophore, was markedly decreased in synapsin I mutant mice. The recovery of synaptic transmission after depletion of neurotransmitter by high-frequency stimulation was greatly delayed. Finally, synapsin I-deficient mice exhibited a strikingly increased response to electrical stimulation, as measured by electrographic and behavioral seizures. These results provide strong support for the hypothesis that synapsin I plays a key role in the regulation of nerve terminal function in mature synapses.

Suppression of synapsin II inhibits the formation and maintenance of synapses in hippocampal culture

Numerous synaptic proteins, including several integral membrane proteins, have been assigned roles in synaptic vesicle fusion with or retrieval from the presynaptic plasma membrane. In contrast, the synapsins, neuron-specific phosphoproteins associated with the cytoplasmic surface of synaptic vesicles, appear to play a much broader role, being involved in the regulation of neurotransmitter release and in the organization of the nerve terminal. Here we have administered antisense synapsin II oligonucleotides to dissociated hippocampal neurons, either before the onset of synaptogenesis or 1 week after the onset of synaptogenesis. In both cases, synapsin II was no longer detectable within 24-48 h of treatment. After 5 days of treatment, cultures were analyzed for the presence of synapses by synapsin I and synaptophysin antibody labeling and by electron microscopy. Cultures in which synapsin II was suppressed after axon elongation, but before synapse formation, did not develop synapses. Cultures in which synapsin II was suppressed after the development of synapses lost most of their synapses. Remarkably, with the removal of the antisense oligonucleotides, neurons and their synaptic connections recovered. These studies lead us to conclude that synapsin II is involved in the formation and maintenance of synapses in hippocampal neurons.