Zic4-Lineage Cells Increase Their Contribution to Visual Thalamic Nuclei during Murine Embryogenesis If They Are Homozygous or Heterozygous for Loss of Pax6 Function

Abstract

Our aim was to study the mechanisms that contribute to the development of discrete thalamic nuclei during mouse embryogenesis (both sexes included). We characterized the expression of the transcription factor coding gene Zic4 and the distribution of cells that expressed Zic4 in their lineage. We used genetic fate mapping to show that Zic4-lineage cells mainly contribute to a subset of thalamic nuclei, in particular the lateral geniculate nuclei (LGNs), which are crucial components of the visual pathway. We observed that almost all Zic4-lineage diencephalic progenitors express the transcription factor Pax6 at variable location-dependent levels. We used conditional mutagenesis to delete either one or both copies of Pax6 from Zic4-lineage cells. We found that Zic4-lineage cells carrying either homozygous or heterozygous loss of Pax6 contributed in abnormally high numbers to one or both of the main lateral geniculate nuclei (LGNs). This could not be attributed to a change in cell production and was likely due to altered sorting of thalamic cells. Our results indicate that positional information encoded by the levels of Pax6 in diencephalic progenitors is an important determinant of the eventual locations of their daughter cells.

The role of the diencephalon in the guidance of thalamocortical axons in mice

“…3A,B). Zic4 is also expressed by some prethalamic cells, with an onset of expression similar to that of Gsx2 (about E9.5; Gaston-massuet et al, 2005), and most diencephalic Zic4 lineage cells express and require Pax6 for their normal development (Li et al, 2018). Using a Zic4 Cre line (Rubin et al, 2011), we observed that prethalamic neurons derived from Zic4 lineage were located in a narrow band close to the thalamic-prethalamic border (Fig.…”

“…We first assessed whether delayed ubiquitous Pax6 deletion, induced in CAG CreER-TM Pax6 fl/fl embryos (referred to here as CAG CreER Pax6 cKOs), disrupts the topography of TCA connections. We induced Cre recombinase activation by tamoxifen administration at E9.5, which caused Pax6 protein loss in CAG CreER Pax6 cKOs from E11.5 onwards (Quintana-Urzainqui et al, 2018), which is when the generation of most thalamic neurons is starting (Li et al, 2018) and before many TCAs have begun to grow (Auladell et al, 2000;López-Bendito and Molnár, 2003). Diencephalic progenitor domains in CAG CreER Pax6 cKOs are fully recognizable (Quintana-Urzainqui Fig.…”

“…The maintenance of order among thalamic axons as they grow is likely to contribute to the generation of orderly topographic mapping in the mature thalamocortical tract. During embryogenesis, thalamic axons exit the thalamus from about E12.5 onwards (Auladell et al, 2000;Braisted et al, 1999;Tuttle et al, 1999), approximately coincident with the cessation of neurogenesis in this structure (Angevine, 1970;Li et al, 2018). They then cross the adjacent prethalamus and turn laterally out of the diencephalon and into the ventral telencephalon where they traverse two consecutive instructive regionsthe corridor (López-Bendito et al, 2006) and the striatumbefore entering the cortex.…”

The role of the diencephalon in the guidance of thalamocortical axons in mice

“…We first assessed whether delayed ubiquitous Pax6 deletion, induced in CAG CreER-TM Pax6 fl/fl embryos (referred to here as CAG CreER Pax6 cKOs), disrupts the topography of TCA connections. We induced Cre recombinase activation by tamoxifen administration at E9.5 which caused Pax6 protein loss in CAG CreER Pax6 cKOs from E11.5 onwards (Quintana-Urzainqui et al, 2018), which is when the generation of most thalamic neurons is starting (Li et al, 2018) and before many TCAs have begun to grow (Auladell and Hans, 2000; López-Bendito and Molnár, 2003). We used both wild type and CAG CreER Pax6 fl/+ littermate embryos as controls since the latter express normal levels of Pax6 protein, almost certainly because of a feedback loop that compensates for a deletion in one allele by increasing the activity of the other (Caballero et al, 2014; Manuel et al, 2015).…”

“…The maintenance of order among thalamic axons as they grow is likely to contribute to the generation of orderly topographic mapping in the mature thalamocortical tract. During embryogenesis, thalamic axons exit the thalamus from about E12.5 onwards (Auladell and Hans, 2000; Braisted et al, 1999; Tuttle et al, 1999), approximately coincident with the cessation of neurogenesis in this structure (Angevine, 1970; Li et al, 2018). They then cross the adjacent prethalamus and turn laterally out of the diencephalon and into the ventral telencephalon where they traverse two consecutive instructive regions - the corridor (Lopez-Bendito et al, 2006) and the striatum - before entering the cortex.…”