N. Agnihotri, A. Jena, M. Moorthy, V. Bhat, J. Sen
The laminar architecture of the mammalian neocortex depends on precise radial migration of newborn neurons to the appropriate cortical layers. This process is governed by the integration of extracellular signals with cell-intrinsic transcriptional programs. BMP signaling has been previously demonstrated to be essential for radial migration of late-born (E15.5) upper-layer cortical neurons. However, the gene expression programs downstream of BMP signaling that regulate this process remained unknown. To address this, we combined temporally targeted in utero electroporation with GeoMx Digital Spatial Profiling (DSP) to map BMP-responsive transcriptional programs in E15.5-born layer II/III neurons at two defined developmental timepoints: E17.5, when neurons actively migrate through the intermediate zone, and postnatal day 0 (P0), when they have completed migration and have attained their laminar position. BMP inhibition produced largely non-overlapping transcriptional changes at these two stages. At E17.5, chromatin-regulatory programs and ribosomal protein gene expression were collectively upregulated upon BMP inhibition. However, by P0, the same cohort of ribosomal genes exhibited downregulation while membrane lipid biosynthesis and synaptic specialization pathways became dominant, revealing a stage-dependent transcriptional switch. A subset of shared BMP-responsive genes was regulated in opposite directions at these two stages, which lent further support to the hypothesis that there is a temporal reorganization of BMP-dependent transcriptional outputs. We selected four candidates from among the BMP-responsive genes for functional studies, namely Mfap4, Olfm2, Adora1, and Arpp21, which belong to diverse functional categories, including extracellular matrix proteins, G protein-coupled receptors, secreted glycoproteins, and RNA-binding proteins. RNAi-mediated knockdown of all four candidates resulted in radial migration defects that closely phenocopied inhibition of BMP signaling, establishing these genes as functional effectors of the BMP signaling pathway regulating neuronal migration.