The Bicoid Class Homeodomain Factors ceh-36/OTX and unc-30/PITX Cooperate in C. elegans Embryonic Progenitor Cells to Regulate Robust Development

Abstract

While many transcriptional regulators of pluripotent and terminally differentiated states have been identified, regulation of intermediate progenitor states is less well understood. Previous high throughput cellular resolution expression studies identified dozens of transcription factors with lineage-specific expression patterns in C. elegans embryos that could regulate progenitor identity. In this study we identified a broad embryonic role for the C. elegans OTX transcription factor ceh-36, which was previously shown to be required for the terminal specification of four neurons. ceh-36 is expressed in progenitors of over 30% of embryonic cells, yet is not required for embryonic viability. Quantitative phenotyping by computational analysis of time-lapse movies of ceh-36 mutant embryos identified cell cycle or cell migration defects in over 100 of these cells, but most defects were low-penetrance, suggesting redundancy. Expression of ceh-36 partially overlaps with that of the PITX transcription factor unc-30. unc-30 single mutants are viable but loss of both ceh-36 and unc-30 causes 100% lethality, and double mutants have significantly higher frequencies of cellular developmental defects in the cells where their expression normally overlaps. These factors are also required for robust expression of the downstream developmental regulator mls-2/HMX. This work provides the first example of genetic redundancy between the related yet evolutionarily distant OTX and PITX families of bicoid class homeodomain factors and demonstrates the power of quantitative developmental phenotyping in C. elegans to identify developmental regulators acting in progenitor cells.

A Conserved Role for Girdin in Basal Body Positioning and Ciliogenesis

“…Moreover, conditional mutants generated via CRISPR-Cas9 gene editing induced in ciliated neurons (Shen et al, 2014) exhibited AWB defects (Figure S2). Finally, AWB defects in grdn-1(ns303) animals were significantly rescued upon expression of grdn-1 under the ceh-37 promoter (Figure 2F) that drives expression transiently, but not exclusively, in the AWB neurons shortly after their birth (Lanjuin et al, 2003; Walton et al, 2015). These results suggest but do not confirm that grdn-1 acts cell-autonomously during early embryogenesis to regulate AWB morphology.…”

Combinatorial decoding of the invariant C. elegans embryonic lineage in space and time

“…It is only when the parallel regulators unc‐120/SRF and hnd‐1/HAND are removed as well that muscle differentiation is completely lost (Fukushige et al ., ). Similarly, in unc‐30;ceh‐36 double mutants, we observed partially penetrant defects in cell division and position as well as expression of the downstream lineage‐specific TF mls‐2/Hmx (Walton et al ., ). This suggests that even essential TFs or TF combinations may have additional redundant partners in any given cellular differentiation decision, thus lethality in mutant animals is a function of the number and identity of cells impacted along with the penetrance of the individual defects.…”

“…Indeed several examples of non‐paralogous parallelism have been identified. We showed that the homeodomain TFs ceh‐36/Otx and unc‐30/Pitx are redundantly required for robust specification of two early embryonic lineages (Walton et al ., ). These TFs are coexpressed early in the ABplp and ABprp lineages and have distinct expression in other lineages.…”

Developmental robustness in the Caenorhabditis elegans embryo

“…The endoderm GRN is therefore robust to genetic variation because many genes make both overlapping and additive contributions to gut specification. Redundancy of this type has been seen in other lineages in the C. elegans embryo, suggesting it is a general strategy for network robustness in cell‐specification GRNs (Good et al, ; Baugh et al, ; Fukushige et al, ; Broitman‐Maduro et al, ; Walton et al, ).…”