Formation of retinal direction-selective circuitry initiated by starburst amacrine cell homotypic contact

Abstract

A common strategy by which developing neurons locate their synaptic partners is through projections to circuit-specific neuropil sublayers. Once established, sublayers serve as a substrate for selective synapse formation, but how sublayers arise during neurodevelopment remains unknown. Here, we identify the earliest events that initiate formation of the direction-selective circuit in the inner plexiform layer of mouse retina. We demonstrate that radially migrating newborn starburst amacrine cells establish homotypic contacts on arrival at the inner retina. These contacts, mediated by the cell-surface protein MEGF10, trigger neuropil innervation resulting in generation of two sublayers comprising starburst-cell dendrites. This dendritic scaffold then recruits projections from circuit partners. Abolishing MEGF10-mediated contacts profoundly delays and ultimately disrupts sublayer formation, leading to broader direction tuning and weaker direction-selectivity in retinal ganglion cells. Our findings reveal a mechanism by which differentiating neurons transition from migratory to mature morphology, and highlight this mechanism’s importance in forming circuit-specific sublayers.

Mapping Transgene Insertion Sites Reveals Complex Interactions Between Mouse Transgenes and Neighboring Endogenous Genes

“…We used TLA to determine insertion sites for three transgenic lines that incorporate fluorescent proteins as reporters: HB9-GFP (green; Wichterle et al, 2002 ), Mito-P (cyan, CFP; Misgeld et al, 2007 ), and TYW3 (yellow, YFP; Kim et al, 2010 ). All label subsets of cells in retina by what appears to be an insertion site-dependent mechanism, and have been used in studies of retinal development and function ( Schubert et al, 2008 ; Kim et al, 2010 ; Trenholm et al, 2011 ; Kay et al, 2011a , b , 2012 ; Duan et al, 2014 ; Krishnaswamy et al, 2015 ; Shekhar et al, 2016 ; Peng et al, 2017 ; Sethuramanujam et al, 2017 ; Ray et al, 2018 ). For two of them, our interest was heightened by breeding experiments in which we attempted to generate transgenic animals that also carried mutations of genes expressed in cell types labeled by the transgene.…”

New Optical Tools to Study Neural Circuit Assembly in the Retina

“…Their interconnections are contained within IPL sublaminae 2 and 4, but each cell invades sublamina 2 and 4 at different times. SACs arrive by P1 (Stacy and Wong, 2003 ; Lefebvre et al, 2015 ; Ray et al, 2018 ), DSGCs by P5 (Kim et al, 2010 ; Ray et al, 2018 ), and BCs at P7–10 (Lefebvre et al, 2012 ; Duan et al, 2014 ). Specific wiring, therefore, does not depend on the synchronous appearance of these partners within the IPL.…”

“…A recent study sheds new light on such sublaminar assembly mechanisms by analyzing the way SACs establish their layers in the developing mouse retina (Ray et al, 2018 ). One population of SACs resides in the INL and extends processes into sublamina 2.…”

“…The other resides in the GCL and extends processes into sublamina 4. Each forms these sublayers at P0-P1, well before their BC and RGC synaptic partners arrive (Stacy and Wong, 2003 ; Sun et al, 2013 ; Ray et al, 2018 ).…”

“…By carefully analyzing the morphology of newly migrated INL SACs, Ray et al ( 2018 ) observed that neighboring SACs contact each other, forming an initial plexus before they extend neurites into the IPL. Such contacts require a transmembrane protein called Megf10, whose loss ablates the inter-SAC contacts and, surprisingly, disrupts SAC lamination in sublamina 2 and 4; a similar disruption of SAC lamination was caused by ablating SACs earlier in development.…”