Kcnab1 Is Expressed in Subplate Neurons With Unilateral Long-Range Inter-Areal Projections

Abstract

Subplate (SP) neurons are among the earliest-born neurons in the cerebral cortex and heterogeneous in terms of gene expression. SP neurons consist mainly of projection neurons, which begin to extend their axons to specific target areas very early during development. However, the relationships between axon projection and gene expression patterns of the SP neurons, and their remnant layer 6b (L6b) neurons, are largely unknown. In this study, we analyzed the corticocortical projections of L6b/SP neurons in the mouse cortex and searched for a marker gene expressed in L6b/SP neurons that have ipsilateral inter-areal projections. Retrograde tracing experiments demonstrated that L6b/SP neurons in the primary somatosensory cortex (S1) projected to the primary motor cortex (M1) within the same cortical hemisphere at postnatal day (PD) 2 but did not show any callosal projection. This unilateral projection pattern persisted into adulthood. Our microarray analysis identified the gene encoding a β subunit of voltage-gated potassium channel ( Kcnab1 ) as being expressed in L6b/SP. Double labeling with retrograde tracing and in situ hybridization demonstrated that Kcnab1 was expressed in the unilaterally-projecting neurons in L6b/SP. Embryonic expression was specifically detected in the SP as early as embryonic day (E) 14.5, shortly after the emergence of SP. Double immunostaining experiments revealed different degrees of co-expression of the protein product Kvβ1 with L6b/SP markers Ctgf (88%), Cplx3 (79%), and Nurr1 (58%), suggesting molecular subdivision of unilaterally-projecting L6b/SP neurons. In addition to expression in L6b/SP, scattered expression of Kcnab1 was observed during postnatal stages without layer specificity. Among splicing variants with three alternative first exons, the variant 1.1 explained all the cortical expression mentioned in this study. Together, our data suggest that L6b/SP neurons have corticocortical projections and Kcnab1 expression defines a subpopulation of L6b/SP neurons with a unilateral inter-areal projection.

Chromatin remodelerArid1aregulates subplate neuron identity and wiring of cortical connectivity

“…Despite the central position of subplate neurons in orchestrating cortical connectivities, the molecular determinants of subplate wiring function remain largely elusive. Previous studies have focused on genes selectively expressed in subplate neurons Oeschger et al, 2012), which are found to mediate subplate neuron specification, migration, and axon development (Arai et al, 2019;Caviness, 1982;Han et al, 2011;Hevner et al, 2001;Kwan et al, 2008;Lai et al, 2008;Rakic et al, 2006;Ratie et al, 2019;Tiong et al, 2019). Here, by cell type-specific dissection of cortical Arid1a function, we uncover a surprisingly selective role for a ubiquitously-expressed chromatin remodeler in establishing the molecular identity and mediating the non-cell autonomous circuit wiring function of subplate neurons.…”

“…Molecular studies have largely focused on subplate-enriched genes Oeschger et al, 2012). Together, these studies have characterized molecular determinants of subplate neuron specification, migration, and axon development (Arai et al, 2019;Caviness, 1982;Han et al, 2011;Hevner et al, 2001;Kwan et al, 2008;Lai et al, 2008;Rakic et al, 2006;Ratie et al, 2019;Tiong et al, 2019), highlighting the complex genetic regulation required for subplate neuron development. The severe axon misrouting phenotypes of subplate ablation (Ghosh et al, 1990;Shatz, 1992, 1993), however, are not broadly recapitulated in these genetic mutants, precluding further mechanistic investigations into the circuit wiring functions of subplate neurons.…”

“…Despite the central role of SPNs in cortical circuit assembly and their potential contribution to neurodevelopmental disorders (5,23,25), they are relatively understudied compared to their cortical plate counterparts. Previous studies have focused on subplate-enriched genes (24,32) and characterized important molecular determinants of SPN specification, migration, and axon projection (33)(34)(35)(36)(37)(38)(39). The severe axon misrouting phenotypes of subplate ablation (18-20) however, are not broadly recapitulated in these genetic mutants.…”

“…Previous studies have focused on genes selectively expressed in SPNs (24,32). These important studies illuminated the genetic bases of SPN specification, migration, and axon development (33)(34)(35)(36)(37)(38)(39). The severe axon misrouting phenotypes of subplate ablation (18)(19)(20), however, are not broadly recapitulated in these genetic mutants.…”

Transient expansion of the expression region of Hsd11b1, encoding 11β‐hydroxysteroid dehydrogenase type 1, in the developing mouse neocortex

“…Expression of the Hsd11b1 gene across 1691 cells isolated from adult mouse S1 was analyzed using public single-cell RNA-seq data (Zeisel et al, 2015) as previously described (Tiong et al, 2019). The annotated expression data (equivalent to the raw data in Gene Expression Omnibus (GEO, https://www.ncbi.nlm.nih.gov/geo/) with the accession GSE60361 except for the cell type annotations) were obtained from the Linnarsson laboratory website (http://linna rsson lab.org/ cortex).…”