Spontaneous Activity Promotes Synapse Formation in a Cell-Type-Dependent Manner in the Developing Retina

Abstract

Spontaneous activity is thought to regulate synaptogenesis in many parts of the developing nervous system. In vivo evidence for this regulation, however, is scarce and comes almost exclusively from experiments in which normal activity was reduced or blocked completely. Thus, whether spontaneous activity itself promotes synaptogenesis or plays a purely permissive role, remains uncertain. In addition, how activity influences synapse dynamics to shape connectivity and whether its effects among neurons are uniform or cell type-dependent is unclear. In mice lacking the cone-rod homeobox gene (Crx), photoreceptors (PRs) fail to establish normal connections with bipolar cells (BCs). Here, we find that retinal ganglion cells (RGCs) in Crx−/− mice become rhythmically hyperactive around the time of eye-opening; as a result of increased spontaneous glutamate release from BCs. This elevated neurotransmission enhances synaptogenesis between BCs and RGCs, without altering the overall circuit architecture. Using live imaging, we discover that spontaneous activity selectively regulates the rate of synapse formation, not elimination, in this circuit. Reconstructions of the connectivity patterns of three BC types with a shared RGC target further revealed that neurotransmission specifically promotes the formation of multisynaptic appositions from one BC type without affecting the maintenance or elimination of connections from the other two. While hyperactivity in Crx−/− mice persists, synapse numbers do not increase beyond four weeks of age, suggesting closure of a critical period for synaptic refinement in the inner retina. Interestingly, despite their hyperactivity, RGC axons maintain normal eye-specific territories and cell type-specific layers in the dorsolateral geniculate nucleus (dLGN).

NGL-2 Regulates Pathway-Specific Neurite Growth and Lamination, Synapse Formation, and Signal Transmission in the Retina

“…Before mating, both mouse lines had been crossed into a C57BL/J6 background for more than five generations. To probe the function of NGL-2 in retinal development, we compared mice lacking the coding exon of the Ngl-2 gene, which is alternatively referred to as Leucine-rich repeat containing 4 (Lrrc4 ) in the literature, in a mixed B6/129S5 background (Ngl-2 Ϫ/Ϫ mice; B6;129S5-Lrrc4 tm1Lex / Mmucd; Mutant Mouse Regional Resource Centers, University of California, Davis, Davis, CA) (Tang et al, 2010) to wild-type littermates (WT). Throughout this study postnatal day 0 (P0) was defined as the day of birth and results from mice of both sexes were combined.…”

“…The density recovery profile (DRP) of HC somata was calculated following the definition of Rodieck (1991) using previously described routines (Soto et al, 2012). Briefly, the density of HCs was measured in 5-m-wide annuli at increasing distance from a reference cell, with each HC in an image serving once as the reference cell.…”

“…To begin to test this hypothesis, we obtained mice lacking the single coding exon of the Ngl-2 gene (Ngl-2 Ϫ/Ϫ mice) (Tang et al, 2010). Toluidine blue-stained semithin sections from P30 mice showed preservation of the retinal cytoarchitecture in Ngl-2 Ϫ/Ϫ mice (Fig.…”

Extrasynaptic glutamate and inhibitory neurotransmission modulate ganglion cell participation during glutamatergic retinal waves

“…Interestingly, recent studies have indicated that the development of some retinal microcircuits may be sensitive to activity, whereas other are not (Kerschensteiner et al 2009;Elstrott and Feller 2010;Xu et al 2010;Soto et al 2012). These data suggest that perhaps as subsets of retinal microcircuits emerge, the maturation of inhibitory synapses exerts a more powerful influence on the structure of correlated activity.…”

“…This spontaneous activity is necessary for refinement of retinal ganglion cell (RGC) axons within target regions of the brain (for reviews, see Guido 2008 andHuberman et al 2008). In addition, there is growing evidence that retinal waves may play a role of the stratification of dendrites of a subset of RGCs (Xu et al 2010) as well as in the synaptic maturation of particular retinal circuits (Kerschensteiner et al 2009;Soto et al 2012).…”