Developmental Mechanism of Tbx15 Expression and Action in the Skin
Our attention to the role of Tbx15 in pigment patterning was motivated by the effects of Agouti in postnatal animals. However, Agouti is also expressed in the embryo, where it provides a convenient marker of ventral dermis identity. Because an expanded domain of embryonic Agouti expression in deH/deH animals is detectable by E14.5, the effects of Tbx15 on dorsoventral patterning must occur prior to this time. Among other T-box genes whose developmental actions are at least partially understood, two general themes have emerged, one focused on the ability to specify alternative fates for an undifferentiated group of precursor cells and another focused on the ability to support proliferative expansion of a cell population whose fate is already determined (reviewed in Tada and Smith 2001). Either mechanism may apply to the apparent dorsal-to-ventral transformation in deH/deH mice. For example, while the expanded domain of Agouti expression in postnatal deH/deH animals can be traced to events that occur between E11.5 and E13.5, the underlying cause may be that embryonic cells in dorsolateral mesenchyme acquire a ventral rather than dorsal identity or that those cells fail to proliferate normally, followed by compensatory expansion of ventral cells. Cell lineage studies should provide a definitive answer, but we favor the latter hypothesis, because measurements of dorsoventral regions according to hair color in deH/deH mice revealed a small increase of the cream-colored ventral region in addition to the approximate doubling of the yellow flank region (see Figure 2).
In embryonic mesenchyme, expression of Tbx15 and Agouti are complementary, and it is possible that Tbx15 acts directly to inhibit Agouti transcription in dorsolateral mesenchyme. However, the ability of Tbx15 to suppress expression of the ventral-specific Agouti isoform in postnatal mice is likely to be indirect, since postnatal expression of Tbx15 occurs broadly along the dorsoventral axis and overlaps extensively with that of Agouti. In either case, the targets of Tbx15 action in the skin include genes in addition to Agouti, since hair length and melanocyte distribution exhibit a demonstrable, albeit subtle, alteration in animals that carry a null Agouti allele. One potential target is Alx4, which, like Agouti, is expressed in ventral embryonic mesenchyme, and, when mutated, affects hair-follicle as well as limb and craniofacial development (Qu et al. 1997, 1998; Wu et al. 2000; Wuyts et al. 2000; Mavrogiannis et al. 2001). However, expression of ventral markers such as Alx4, as well as Alx3 and Msx2, appears to be unaffected at E11.5 in deH/deH embryos (data not shown).