Spatiotemporal Patterning of ABC Multidrug‐Resistance Transporters During Germ Layer Specification

Abstract

Xenobiotic transporters are important for the protection of differentiated somatic cells, yet less is known about the role of these proteins in rapidly changing embryonic cells. A surprisingly large number of ABC transporters (80%) are expressed from fertilized egg to gastrulation, but their spatial and temporal patterns of expression during early embryogenesis remain incompletely understood. Here I report a pattern of ABC transporter expression that emerges during the early differentiation of mesoderm, endoderm, and ectoderm of the sea urchin. These included orthologs of six major xenobiotic transporters, including ABCB1, ABCB4, ABCC1, ABCC4, ABCC5, and ABCG2. In‐situ hybridization (ISH) revealed that transporters followed one of three patterns of early developmental expression: The first, seen with protective and homeostatic genes such as ABCB1 and ABCC1, was the early and ubiquitous expression across all three germ layers, and later enrichment in ectoderm‐derived sensory and feeding structures of the larva. The second, seen with the transporters ABCC4 and ‐C5 that are known to transport signal molecules, is a spatially restricted expression domain within a ring of mesodermal cells of the early blastula; ABCC4 then becomes restricted to primordial germ cells (PGCs) and their embryonic niche during gastrulation. Third, ABCB4 and ABCG2 show expression only in endoderm fated cells, particularly in the hindgut (presumptive intestines) at the onset of gut development; both genes eventually expand to the midgut region (presumptive stomach). To further refine the ISH map, I used fluorescent substrates to assay the functional outputs of transporter mRNA localization. ABCC1 and ‐C4 specific substrates are strongly effluxed out of the ring of mesodermal cells. This overlaps with robust ABCC5 transport activity in blastulae, suggesting a potential ABCC‐mediated signaling center in early mesoderm, where the PGCs are born. In the endoderm, ABCB4 and ABCG2‐specific efflux activity begins several days before the larva feeds. Collectively, this work is the first systems‐level description of the emergence of xenobiotic transporter expression patterns across the germ layers during embryogenesis. The resulting map of transport reveals unanticipated developmental functions for these genes and provides a framework for understanding the genetic regulatory controls of transporter expression. This will have additional impact on predicting and mitigating the effects of xenobiotic exposures during vulnerable windows of early development.Support or Funding InformationSupported by NIH R01ES027921 and R01ES030318 to AH, and NIH F32ES029843 to CSS.