Vsx1 Transiently Defines an Early Intermediate V2 Interneuron Precursor Compartment in the Mouse Developing Spinal Cord

Abstract

Spinal ventral interneurons regulate the activity of motor neurons, thereby controlling motor activities. Interneurons arise during embryonic development from distinct progenitor domains distributed orderly along the dorso-ventral axis of the neural tube. A single ventral progenitor population named p2 generates at least five V2 interneuron subsets. Whether the diversification of V2 precursors into multiple subsets occurs within the p2 progenitor domain or involves a later compartment of early-born V2 interneurons remains unsolved. Here, we provide evidence that the p2 domain produces an intermediate V2 precursor compartment characterized by the transient expression of the transcriptional repressor Vsx1. These cells display an original repertoire of cellular markers distinct from that of any V2 interneuron population. They have exited the cell cycle but have not initiated neuronal differentiation. They coexpress Vsx1 and Foxn4, suggesting that they can generate the known V2 interneuron populations as well as possible additional V2 subsets. Unlike V2 interneurons, the generation of Vsx1-positive precursors does not depend on the Notch signaling pathway but expression of Vsx1 in these cells requires Pax6. Hence, the p2 progenitor domain generates an intermediate V2 precursor compartment, characterized by the presence of the transcriptional repressor Vsx1, that contributes to V2 interneuron development.

Zygotic Vsx1 Plays a Key Role in Defining V2a Interneuron Sub-Lineage by Directly Repressing tal1 Transcription in Zebrafish

“…During the diversification of V2a and V2b cells, vsx2 is steadily expressed in V2a but not in tal1-expressing V2b cells in all the examined vertebrates [11,22]. In zebrafish embryonic spinal cords, the expression of vsx1 initiated in the common P2 progenitors is transiently maintained in the V2a sub-lineage, and a significant number of V2a cells co-express vsx1 and vsx2 in wild-type embryos [11], suggesting that vsx2 and vsx1 are expressed in the same cells but in a temporally distinct manner.…”

“…It has been observed that Vsx1 can inhibit vsx2 in the type 7 cone bipolar cells of mice [29]. However, Vsx2 distribution is not expanded in the absence of Vsx1 in mouse spinal cord [22]. In zebrafish Notch signaling mutants, the numbers of both vsx2-expressing cells and vsx1-expressing cells are increased [11].…”

“…It has been observed that Lhx3 is able to promote Vsx2 expression in the developing spinal cord [17,18]. However, in normal development of spinal cord, Vsx2 is expressed only in the progeny cells of P2 progenitors that co-express Vsx1 and Lhx3 [9][10][11]22] but not in the progeny cells of motor neuron progenitors in which lhx3 is not expressed with vsx1 [11,22]. These phenomena suggest that Lhx3 alone is unable to promote Vsx2 expression.…”

“…Previous studies have demonstrated that Vsx1 can directly repress target genes expression [23,24]. In all the examined vertebrates, Vsx1 is expressed in the P2 precursors, while Vsx2 is expressed in mature V2a cells [11,12,[19][20][21][22]. After the final mitotic division of V2 precursors, the expression of Vsx1 is maintained in V2a but not in V2b cells at the early diversification stage in zebrafish [11,12].…”

“…A paired-like transcription factor Vsx1, containing an evolutionary conservative homeodomain and CVC domain, is the paralog of Vsx2 [19][20][21][22]. Previous studies have demonstrated that Vsx1 can directly repress target genes expression [23,24].…”