Description

The UNC-62 homeodomain protein regulates AWC general identity and subsequently plays a cell autonomous role, determined by mosaic analysis, in AWC asymmetry during embryogenesis (Hsieh et al., 2021). An integrated unc-62::GFP fosmid transgene, in which all UNC-62 protein isoforms are tagged with GFP at the C- terminus (Van Nostrand et al., 2013) (Figure 1A), rescued unc-62(lf) mutant phenotypes of AWC general identity, determined by odr-1p::DsRed expression, and AWC asymmetry, determined by str-2p::GFP expression (Hsieh et al., 2021). These results suggest that UNC-62::GFP fusion protein expressed from the unc-62::GFP fosmid transgene is functional for AWC development. It has been shown that this integrated unc-62::GFP fosmid transgene is expressed in sensory neurons, touch neurons, interneurons, ventral nerve cord motor neurons, and head motor neurons, but it is not expressed in AWC in late-stage larvae or young-stage adult worms using the multicolor transgene NeuroPAL (Reilly et al., 2020).

The AWC neurons are born near the end of gastrulation; AWC asymmetry is established around the 1.5-fold and 3-fold embryonic stage (Sulston et al., 1983; Chuang and Bargmann, 2005). To determine whether unc-62 is expressed in AWC neurons at the embryonic stages of AWC development, the expression pattern of the integrated unc-62::GFP fosmid transgene (Van Nostrand et al., 2013) (Figure 1A), was analyzed with integrated hlh-16::H1-wCherry or odr-1p::TagRFP transgene, early or late AWC marker, respectively. UNC-62::GFP was not detected in AWC neurons at the end of gastrulation, 1.5-fold, or 3-fold embryos (Figure 1B-D). Consistent with our results, single-cell RNA sequencing data revealed a very low expression level of unc-62 in AWC during early embryogenesis as well as an undetectable level of unc-62 in AWC in the later embryonic stage and second-larval stage (Cao et al., 2017; Packer et al., 2019). Together, these results suggest that unc-62 may be expressed at an undetectable level and/or UNC-62 protein may have a very short half-life in embryonic AWC neurons.