Stereotyped Axon Pruning via Plexin Signaling Is Associated with Synaptic Complex Elimination in the Hippocampus

Abstract

Plexin signaling is required for stereotyped pruning of long axon collaterals in the vertebrate CNS; however, a cellular basis for plexins on stereotyped pruning has not been determined. Using quantitative electron microscopy and immunocytochemistry, we found that infrapyramidal mossy fiber axon collaterals form transient synaptic complexes with basal dendrites of CA3 pyramidal cells in the early postnatal mouse hippocampus. At later postnatal ages, these synaptic complexes stop maturing and are removed before stereotyped pruning by a mechanism that does not involve axon degeneration and glial cell engulfment. In knock-out mice that lack plexin-A3 signaling, the synaptic complexes continue to mature, and, as a result, the collaterals are not pruned. Thus, our results suggest that intact plexin-A3 signaling contributes to synaptic complex elimination, which is associated with stereotyped axon pruning.

Development of hippocampal mossy fiber synaptic outputs by new neurons in the adult brain

“…mossy boutons that contact interneurons are generally small in size and contain one synapse. In contrast, the synaptic complexes analyzed in this study are composed of a large presynaptic mossy fiber bouton and postsynaptic dendritic excrescence from a CA3 pyramidal cell with multiple synaptic contacts forming between the two (16,17,29). Given the small percentage of new neurons among all of the existing mature DGCs in the adult hippocampus, unlabeled mossy fiber boutons adjacent to GFP ϩ boutons were analyzed and used as a control for mature bouton structure.…”

“…The changes in bouton structure over development were assessed according to a classic staging paradigm (16,17) (see Methods). Briefly, stage 1 boutons form either no or few synaptic contacts exclusively onto dendritic shafts (nonspinous synaptic contacts), stage 2 boutons have synaptic contacts onto immature dendritic spines (finger-like structures without discernible organelles), and stage 3 boutons have synaptic contacts onto mature dendritic spines containing organelles, with stage 3a boutons having fewer synaptic contacts onto each dendritic spine (Ͻ2 synaptic contacts per spine) than stage 3b boutons (Ն2 synaptic contacts per spine).…”

“…Newborn dentate granule cells (DGCs) in the adult hippocampus are known to extend their axons into the CA3 subfield (8,13,14), but it is unclear whether typical synaptic structures are formed by these new neurons in vivo. Using a retrovirus expressing EGFP to birth-date and label adultborn neurons (5,8,15), we examined the development of their axons and synaptic outputs by confocal and quantitative immunoelectron microscopy (EM) with 3D reconstruction of labeled boutons (15)(16)(17). We show that the axons of adult-born DGCs form typical mossy fiber bouton synaptic complexes with CA3 pyramidal cells within 8 weeks.…”

Guidance Molecules in Axon Pruning and Cell Death

“…The transient long IPB is stereotypically pruned back later in development. Electron microscopic analysis suggests that the IPB pruning is mediated by axon branch retraction (Liu et al 2005), and the length of the IPB is reported to be affected …”

“…Quantitative immuno-electron microscopy has shown that the IPB axons form transient synaptic contacts in their transient target region before they are pruned (Liu et al 2005). The pruning of the IPB is preceded by the elimination of these immature synaptic contacts, which is triggered by the action of Sema3F signaling.…”