Maturation of complex synaptic connections of layer 5 cortical axons in the posterior thalamic nucleus requires SNAP25

Abstract

Synapses are able to form in the absence of neuronal activity, but how is their subsequent maturation affected in the absence of regulated vesicular release? We explored this question using 3D electron microscopy and immuno electron microscopy analyses in the large, complex synapses formed between cortical sensory efferent axons and dendrites in the posterior thalamic nucleus. Using a Snap25 conditional knockout we found that during the first two postnatal weeks the axonal boutons emerge and increase in the size similar to the control animals. However, by P18, when an adult-like architecture should normally be established, axons were significantly smaller with 3D reconstructions showing that each Snap25-cko bouton only forms a single synapse with the connecting dendritic shaft. No excrescences from the dendrites were formed, and none of the normally large glomerular axon endings were seen. These results show that activity mediated through regulated vesicular release from the presynaptic terminal is not necessary for the formation of synapses, but it is required for the maturation of the specialised synaptic structures between layer 5 corticothalamic projections in Po.

Transcriptional networks identify synaptotagmin-like 3 as a regulator of cortical neuronal migration during early neurodevelopment

“…However, vesicle trafficking, docking, and fusion are indispensable for neurotransmitter release during neural development. Gene functional studies in vesicular trafficking have shown that gene knockout or condi-tional knockout of SNAP25 and synaptobrevin, SNARE complex key components that mainly regulating vesicle fusion, eliminated evoked neurotransmitter release and reduced spontaneous miniature excitatory postsynaptic currents and synaptic maturation in the mouse brain (Hayashi et al, 2020;Hoerder-Suabedissen et al, 2019;Molná r et al, 2002;Verhage et al, 2000;Washbourne et al, 2002). Furthermore, mutation of synaptotagmin-1, a Ca 2+ sensor localized on synaptic vesicles, and C-type tandem C2 protein family member (Fukuda and Mikoshiba, 2001), can alter spontaneous and evoked neurotransmitter release (Xu et al, 2009).…”

A combinatorial input landscape in the “higher-order relay” posterior thalamic nucleus

“…Tg(Rbp4cre)KL100Gsat/Mmucd (Rbp4-Cre; Jackson Laboratories) mice were crossed with B6;129S6-Gt(ROSA)26Sortm14(CAG-tdTomato)Hze/J (Ai14) to constitutively drive fluorescent protein expression in cortical L5 pyramidal neurons (Hayashi et al, 2021).…”

“…The Posterior (Po) nucleus of the mouse thalamus is a model HO nucleus because it receives a robust L5b input and mediates transthalamic communication (Theyel et al, 2010;Guo et al, 2020). This L5b projection has been widely described for the barrel field of the primary somatosensory cortex (S1BF; Bourassa et al, 1995;Veinante et al, 2000b;Killackey and Sherman, 2003;Sumser et al, 2017;Hoerder-Suabedissen et al, 2018;Hayashi et al, 2021). Terminals with driver characteristics in Po can also originated from other cortical structures, such as secondary somatosensory (S2; Liao et al, 2010) and motor cortices (Rouiller et al, 1991;Sampathkumar et al, 2021).…”

The role of snare proteins in cortical development

“…However, when Snap25 expression is abolished specifically in layer 5 neurons from birth, these specialized synapses between the giant terminals of the cortical axons with the posterior thalamic neurons show altered development, with the thalamic dendritic protrusions failing to mature into the giant interdigitating shape. This suggests that Snap25‐mediated synaptic vesicle release by cortical layer 5 neurons is not crucial for initial formation of synapses but is fundamental for the maturation of specialized synapses (Hayashi et al., 2021) (Figure 5).…”

“…See Hayashi et al. (2021). Maturation of complex synaptic connections of layer 5 cortical axons in the posterior thalamic nucleus requires Snap25.…”