Heme homeostasis is regulated by the conserved and concerted functions of HRG-1 proteins

Hemes are essential but potentially cytotoxic cofactors that participate in critical and diverse biological processes. Although the pathway and intermediates for heme biosynthesis have been well defined, the intracellular networks which mediate heme trafficking remain unknown. Caenorhabditis elegans and related helminths are natural heme auxotrophs requiring environmental heme for growth and development. We exploited this auxotrophy to identify HRG-1 and HRG-4 in C. elegans and show that they are essential for heme homeostasis and normal vertebrate development. We demonstrate that heme deficiency upregulates expression of hrg-4 and its evolutionarily conserved paralog hrg-1. Depletion of either HRG-1 or HRG-4 in worms results in disruption of organismal heme sensing and abnormal response to heme analogs. HRG-1 and HRG-4 are novel transmembrane proteins that bind heme and have evolutionarily conserved functions. Transient knockdown of hrg-1 in zebrafish leads to hydrocephalus, yolk tube malformations, and, most strikingly, profound defects in erythropoiesis - phenotypes that are fully rescued by worm HRG-1. These findings reveal unanticipated and conserved pathways for cellular heme trafficking in animals that defines the paradigm for eukaryotic heme transport. Uncovering the mechanisms of heme transport in C. elegans will provide novel insights into human disorders of heme metabolism and generate unique anthelmintics to combat worm infestations. Keywords: dose-response血红素（heme）是一类必需但具有潜在细胞毒性的辅因子，参与诸多关键且多样的生物学过程。尽管血红素生物合成的通路及中间产物已得到明确阐释，但介导血红素转运的细胞内网络仍未可知。秀丽隐杆线虫（Caenorhabditis elegans）及其相关蠕虫为天然血红素营养缺陷型生物，其生长发育依赖环境中的血红素。本研究利用该营养缺陷型特性，在秀丽隐杆线虫中鉴定出HRG-1与HRG-4，并证实二者对于血红素稳态及正常脊椎动物发育至关重要。研究表明，血红素缺乏会上调hrg-4及其进化保守的旁系同源基因hrg-1的表达。在蠕虫中敲除HRG-1或HRG-4，会破坏机体的血红素感知能力，并导致其对血红素类似物的响应异常。HRG-1与HRG-4为一类新型跨膜蛋白，可结合血红素且具备进化保守的功能。在斑马鱼中瞬时敲低hrg-1，会引发脑积水、卵黄管畸形，最显著的是会造成严重的红细胞生成缺陷——这类表型可通过蠕虫HRG-1完全挽救。这些研究结果揭示了动物细胞中此前未被发现的、保守的血红素转运通路，该通路确立了真核生物血红素转运的研究范式。解析秀丽隐杆线虫中的血红素转运机制，将为人类血红素代谢紊乱相关疾病提供全新的研究视角，并有望开发出靶向蠕虫侵染的新型驱虫药物。关键词：剂量响应（dose-response）