Basal Protrusions Mediate Spatiotemporal Patterns of Spinal Neuron Differentiation

Abstract

Summary During early spinal cord development, neurons of particular subtypes differentiate with a sparse periodic pattern while later neurons differentiate in the intervening space to eventually produce continuous columns of similar neurons. The mechanisms that regulate this spatiotemporal pattern are unknown. In vivo imaging in zebrafish reveals that differentiating spinal neurons transiently extend two long protrusions along the basal surface of the spinal cord before axon initiation. These protrusions express Delta protein, consistent with the hypothesis they influence Notch signaling at a distance of several cell diameters. Experimental reduction of Laminin expression leads to smaller protrusions and shorter distances between differentiating neurons. The experimental data and a theoretical model support the proposal that neuronal differentiation pattern is regulated by transient basal protrusions that deliver temporally controlled lateral inhibition mediated at a distance. This work uncovers a stereotyped protrusive activity of newborn neurons that organize long-distance spatiotemporal patterning of differentiation.

Delta-Notch Signaling: The Long and the Short of a Neuron’s Influence on Progenitor Fates

“…We provided evidence that basal protrusions developed by differentiating neurons may spatially and temporally regulate the pattern of neuronal differentiation through long-range Delta-Notch-mediated lateral inhibition. This was further confirmed by mathematical modelling that showed the positioning and timing of neuronal differentiation cannot be explained by Delta-Notch signalling occurring between immediate neighbours but can be explained by the basal protrusions delivery of long-range Delta-Notch-mediated lateral inhibition [54].…”

“…Although it conventionally occurs between immediate neighbours, in Drosophila, Notch-Delta signalling has been shown to operate over larger distances to pattern mechanosensory bristles [51][52][53][54][55][56]. We recently showed that this can occur in the vertebrate neural tube [54]. All recently born neurons in the zebrafish spinal cord extend two long protrusions along the basal surface of the spinal cord that span several neural progenitors ( Figure 2).…”

“…All recently born neurons in the zebrafish spinal cord extend two long protrusions along the basal surface of the spinal cord that span several neural progenitors ( Figure 2). These basal protrusions express high levels of Delta protein and Notch reporter activation occurs in the cells within their reach, suggesting that basal protrusions regulate Delta-Notch signalling pathway activation over long distances [54]. Spinal neurons initially differentiate with a sparse, periodic pattern [37,[57][58][59][60] and never differentiate close in space and time [54].…”

“…These basal protrusions express high levels of Delta protein and Notch reporter activation occurs in the cells within their reach, suggesting that basal protrusions regulate Delta-Notch signalling pathway activation over long distances [54]. Spinal neurons initially differentiate with a sparse, periodic pattern [37,[57][58][59][60] and never differentiate close in space and time [54]. We provided evidence that basal protrusions developed by differentiating neurons may spatially and temporally regulate the pattern of neuronal differentiation through long-range Delta-Notch-mediated lateral inhibition.…”

“…The subcellular localisation of Delta-Notch signalling in the majority of the contexts remains poorly characterised at the cellular and subcellular level. Depending on the vertebrate system and the moment of the cell cycle, Delta-Notch interactions have been suggested to occur closer to the apical surface [32], at the cell body and at the basal surface of the neuroepithelium [54].…”