Differential dynamics of cortical neuron dendritic trees revealed by long-term in vivo imaging in neonates

Shingo Nakazawa,Hidenobu Mizuno,Takuji Iwasato

doi:10.1038/s41467-018-05563-0

Abstract

Proper neuronal circuit function relies on precise dendritic projection, which is established through activity-dependent refinement during early postnatal development. Here we revealed dynamics of dendritic refinement in the mammalian brain by conducting long-term imaging of the neonatal mouse barrel cortex. By “retrospective” analyses, we identified “prospective” barrel-edge spiny stellate (SS) neurons in early neonates, which had an apical dendrite and primitive basal dendrites (BDs). These neurons retracted the apical dendrite gradually and established strong BD orientation bias through continuous “dendritic tree” turnover. A greater chance of survival was given to BD trees emerged in the barrel-center side, where thalamocortical axons (TCAs) cluster. When the spatial bias of TCA inputs to SS neurons was lost, BD tree turnover was suppressed, and most BD trees became stable and elaborated mildly. Thus, barrel-edge SS neurons could establish the characteristic BD projection pattern through differential dynamics of dendritic trees induced by spatially biased inputs.

Highlights

Proper neuronal circuit function relies on precise dendritic projection, which is established through activity-dependent refinement during early postnatal development
As a first step toward in vivo imaging of dendritic refinement in the mammalian brain, we recently developed two methods: (1) the “Supernova” system, which allows sparse yet bright in vivo labeling of cortical layer 4 (L4) neurons when used in combination with in utero electroporation-based gene transfection[29,30], and (2) TCAGFP transgenic (Tg) mice, which allows in vivo labeling of TCAs30
In L4 of the developing mouse barrel cortex, whisker-specific thalamocortical axons (TCAs) clusters emerge in the barrel center around P330, and eSS neurons acquire their characteristic basal dendrites (BDs), which are asymmetrically extended within single barrels, primarily by P630,32

Summary

Introduction

Proper neuronal circuit function relies on precise dendritic projection, which is established through activity-dependent refinement during early postnatal development. Major difficulties include (1) sparse yet intense in vivo labeling of neurons, which is necessary for visualization of detailed dendritic morphology, (2) in vivo labeling of specific axons that are presynaptic to dendrites of an identified neuron, and (3) use and maintenance of fragile newborn mice during in vivo imaging sessions For these reasons, instead of in vivo imaging, acute or chronic slice culture has been predominantly used for two-photon or confocal TL imaging of dendritic development (e.g., dendritic arborization) in the mammalian cortex[12], hippocampus[13,14], and cerebellum[15]. SS neurons located around the barrel edge (edge-SS or eSS neurons) extend their basal dendrites (BDs) asymmetrically toward the barrel center, where termini of thalamocortical axons (TCAs) transmitting information from a single whisker form a cluster[24,25] These characteristic barrel morphologies are formed during the first postnatal week in a periphery-derived input-dependent manner[26,27,28], making them a key model of the developmental refinement of cortical circuits. To fully understand dynamic mechanisms of BD refinement of SS neurons, long-term (over days) imaging starting at earlier neonatal stages was awaited

Methods

Results

Conclusion