Shh and ZRS enhancer co-localisation is specific to the zone of polarizing activity
Limb-specific Shh expression is regulated by the (~1 Mb distant) ZRS enhancer. In the mouse, limb bud restricted spatiotemporal expression of Shh occurs from ~E10-E11.5 at the distal posterior margin and is essential for correct autopod formation. Here, we have analysed the higher-order chromatin conformation of Shh in expressing and non-expressing tissues, both by fluorescence in situ hybridisation (FISH) and by chromosome conformation capture (5C). Conventional and super-resolution light microscopy identified significantly elevated frequencies of Shh/ZRS co-localisation only in the Shh expressing regions of the limb bud. However, Shh-ZRS spatial distances were consistently shorter than intervening distances to a neural enhancer in all tissues and developmental stages analysed. 5C identified a topologically associating domain (TAD) over the Shh/ZRS genomic region and enriched interactions between Shh and ZRS throughout E11.5 embryos. Shh/ZRS co-localisation, therefore, correlates with the spatiotemporal domain of limb bud-specific Shh expression, but close Shh/ZRS proximity in the nucleus occurs regardless of whether the gene or enhancer is active. We suggest that this constrained chromatin configuration optimises the opportunity for the active enhancer to locate and instigate Shh expression.