Development and elimination of presynaptic elements on polylysine-coated beads implanted in neonatal rat cerebellum

Towards an Understanding of Synapse Formation

Synapses are intercellular junctions specialized for fast, point-to-point information transfer from a presynaptic neuron to a postsynaptic cell. At a synapse, a presynaptic terminal secretes neurotransmitters via a canonical release machinery, while a postsynaptic specialization senses neurotransmitters via diverse receptors. Synaptic junctions are likely organized by trans-synaptic cell-adhesion molecules (CAMs) that bidirectionally orchestrate synapse formation, restructuring, and elimination. Many candidate synaptic CAMs were described, but which CAMs are central actors and which are bystanders remains unclear. Moreover, multiple genes encoding synaptic CAMs were linked to neuropsychiatric disorders, but the mechanisms involved are unresolved. Here, I propose that engagement of multifarious synaptic CAMs produces parallel trans-synaptic signals that mediate the establishment, organization, and plasticity of synapses, thereby controlling information processing by neural circuits. Among others, this hypothesis implies that synapse formation can be understood in terms of inter- and intracellular signaling, and that neuropsychiatric disorders involve an impairment in such signaling.

β-Pix modulates actin-mediated recruitment of synaptic vesicles to synapses

Presynaptic compartments are formed through the recruitment of preassembled clusters of proteins to points of cell-cell contact, however, the molecular mechanism(s) underlying this process remains unclear. We demonstrate that clusters of polymerized actin can recruit and maintain synaptic vesicles to discrete sites along the axon, and that cadherin/β-catenin/scribble/β-pix complexes play an important role in this event. Previous work has demonstrated that β-catenin and scribble are important for the clustering of vesicles at synapses. We demonstrate that β-pix, a Rac/Cdc42 guanine nucleotide exchange factor (GEF), forms a complex with cadherin, β-catenin, and scribble at synapses and enhances localized actin polymerization in rat hippocampal neurons. In cells expressing β-pix siRNA or dominant-negative β-pix that lacks its GEF activity, actin polymerization at synapses is dramatically reduced, and synaptic vesicle localization is disrupted. This β-pix phenotype can be rescued by cortactin overexpression, suggesting that β-pix-mediated actin polymerization at synapses regulates vesicle localization.

Regenerating crayfish motor axons assimilate glial cells and sprout in cultured explants

Phasic and tonic motor nerves originating from crayfish abdominal ganglia, in 2-3-day-old cultured explants, display at their transected distal ends growth zones from which axonal sprouts arise. The subcellular morphology of this regenerative response was examined with thin serial-section electron microscopy and reveals two major remodeling features. First, the external sprouts that exit the nerve are a very small part of a much more massive sprouting response by individual axons comprising several orders of internal sprouts confined to the nerve. Both internal and external sprouts have a simple construction: a cytoskeleton of microtubules and populations of mitochondria, clear synaptic vesicles, membranous sacs, and extrasynaptic active zone dense bars, features reminiscent of motor nerve terminals. Close intermingling of the sprouts of several axons give rise to a neuropil-like arbor within the nerve. Thus, extensive sprouting is an intrinsic response of crayfish motor axons to transection. Second, an equally dramatic remodeling feature is the appearance of nuclei, which resemble those of adjacent glial cells, within the motor axons. These nuclei often appear where the adjoining membranes of the axon and glial cell are disrupted and where free-standing lengths of the double membrane are present. These images signify a breakdown of the dividing membranes and assimilation of the glial cell by the axon, the nucleus being the most visible sign of such assimilation. Thus, crayfish motor axons respond to transection by assimilating glial cells that may provide regulatory and trophic support for the sprouting response.

Highly basic 30- and 32-kilodalton proteins associated with synapse formation on polylysine-coated beads in enriched neuronal cell cultures

Neuronal proteins involved in axonal outgrowth and synapse formation were examined in an enriched neuronal cell culture system of the cerebellum. In rat cerebellar cell cultures, 98.9% of the cells are neurons and the remaining 1.1% of the cells are flat nonneuronal cells. These enriched neuronal cultures, examined with two-dimensional gel electrophoresis, showed protein patterns similar to those of neonatal cerebellum, but very different patterns from glial enriched cultures. High levels of a neuronal membrane acidic 29-kilodalton (kD) protein were found. It has been shown previously that neuronal cultures incubated with polylysine-coated beads will develop numerous presynaptic elements on the bead surface. We report here that isolation of the beads from enriched neuronal cell cultures incubated with [35S]methionine showed, with two-dimensional nonequilibrium pH gradient gel electrophoresis (2D-NEPHGE), levels of a basic 32-kD protein (pI 8) note detected in cultures alone, and increased levels of a 30-kD protein (pI 10). When culture medium was examined with 2D-NEPHGE, three acidic proteins were identified that were secreted by the cultured neurons. In summary, a neuronal enriched cell culture system was used with isolated polylysine-coated beads to identify basic 30-kD and 32-kD proteins that may be involved in synapse formation.

Presynaptic elements on artificial surfaces. A model for the study of development and regeneration of synapses

Recently a model has been developed to study the synapse formation in which the components of a synapse can be isolated and examined independently. The observation of neurites forming presynaptic elements on polylysine-coated surfaces is a model for which the formation of presynaptic elements can be studied independently of a cellular postsynaptic element. Studies with neurons from both cell cultures and the intact cerebellum have shown that beads coated with poly-basic proteins can serve as a "postsynaptic element." With use of this system, observation have shown that the presynaptic element can form quickly, within 3 h, and contain many of the characteristics of a mature presynaptic element, such as synaptic vesicle antigens. Additional studies have shown that astrocytes appear to be involved in the loss or removal of the presynaptic elements on beads. Thus, synaptogenesis may involve the development of inappropriate synaptic contacts, which are eliminated by astrocytes. The lack of regeneration in the central nervous system (CNS) also may involve the astrocyte's ability to remove immature and/or inappropriate presynaptic elements and growth cones as they attempt to cross the lesion site.

Presynaptic elements formed on polylysine-coated beads contain synaptic vesicle antigens

Cell cultures of the rat cerebellum were immunostained with antibodies to synaptic vesicle antigens, Synapsin I and SV48. Light microscopic immunocytochemistry showed that the initial appearance of demonstrable SV48 and Synapsin I immunoreactivity occurred at different times. Synapsin I immunostaining, unlike SV48 immunostaining, was first seen at 3 days in vitro as occasional punctate immunofluorescence in neurites, while SV48 immunostaining was first seen at 5 days in vitro. Both SV48 and Synapsin I punctate immunostaining became frequent at 7 days in vitro. Double labelling experiments showed coexistence of the above proteins in punctate swellings and growth cones. Using the electron microscope, either SV48 or Synapsin I immunostaining was demonstrated within presynaptic elements in the neuropil. When cultures were incubated with polylysine-coated beads, both types of immunostaining were found in the vesicle containing presynaptic elements formed on the bead surface. It is concluded that Synapsin I and SV48 are co-localized in the same populations of presynaptic elements, co-localized in some growth cones and found in presynaptic elements on beads.