Molecular Analyses of V0v Spinal Interneurons and Identification of Transcriptional Regulators Downstream of Evx1 and Evx2 in these Cells

AbstractBackgroundV0v spinal interneurons are highly conserved, glutamatergic, commissural neurons that function in locomotor circuits. We have previously shown that Evx1 and Evx2 are required to specify the neurotransmitter phenotype of these cells. However, we still know very little about the gene regulatory networks that act downstream of these transcription factors in V0v cells.MethodsTo identify candidate members of V0v gene regulatory networks, we FAC-sorted WT andevx1;evx2double mutant zebrafish V0v spinal interneurons and expression-profiled them using microarrays and single cell RNA-seq. We also usedin situhybridization to compare expression of a subset of candidate genes inevx1;evx2double mutants and wild-type siblings.ResultsOur data reveal two molecularly distinct subtypes of V0v spinal interneurons at 48 h and suggest that, by this stage of development,evx1;evx2double mutant cells transfate into either inhibitory spinal interneurons, or motoneurons. Our results also identify 25 transcriptional regulator genes that require Evx1/2 for their expression in V0v interneurons, plus a further 11 transcriptional regulator genes that are repressed in V0v interneurons by Evx1/2. Two of the latter genes arehmx2andhmx3a. Intriguingly, we show that Hmx2/3a, repress dI2 interneuronal expression ofskor1aandnefma, two genes that require Evx1/2 for their expression in V0v interneurons. This suggests that Evx1/2 might regulateskor1aandnefmaexpression in V0v interneurons by repressing Hmx2/3a expression.ConclusionsThis study identifies two molecularly distinct subsets of V0v spinal interneurons, as well as multiple transcriptional regulators that are strong candidates for acting downstream of Evx1/2 to specify the essential functional characteristics of these cells. Our data further suggest that in the absence of both Evx1 and Evx2, V0v spinal interneurons initially change their neurotransmitter phenotypes from excitatory to inhibitory and then, later, start to express markers of distinct types of inhibitory spinal interneurons, or motoneurons. Taken together, our findings significantly increase our knowledge of V0v and spinal development and move us closer towards the essential goal of identifying the complete gene regulatory networks that specify this crucial cell type.