Long-range signaling at the neural-intestinal axis promotes organismal heme homeostasis

Purpose: The goal of this study is to understand how dbl-1, which is made primarily in neurons, and hrg-7, which is exclusively made in the intestine, contribute to systemic heme homeostasis. Methods: mRNA profiles of late L4 dbl-1(nk3) and hrg-7(tm6801) mutant C. elegans fed OP50 E. coli or OP50 + 50µM heme were compared to mRNA profiles from wildtype (WT) broodmates. Profiles were generated with single-end 50 base reads obtained using Illumina’s HiSeq 2500. Bioinformatics quality control was performed followed by alignment of reads to the ce10 reference genome using Tophat2, version 2.1.0. We found differentially expressed genes using Cufflinks 2, version 2.2.1 with a cutoff of 0.05 on False Discovery Rate (FDR). Results: We found a substantial overlap of genes regulated by both dbl-1 and hrg-7, including 49 heme-responsive genes (hrgs) in low heme (OP50) and 11 hrgs in high heme (OP50 + 50µM). Additionally, our data indicate crosstalk between dbl-1 and hrg-7 signaling. dbl-1 directly regulates hrg-7 expression, while hrg-7 regulates three components of the dbl-1 signaling pathway. Conclusions: Our study demonstrates that communication between the neuron and intestine is essential for heme homeostasis. Specifically, we report that HRG-7 functions as a secreted signaling factor which communicates intestinal heme status with extraintestinal tissues by integrating a DBL-1/BMP -dependent response from the neurons to transcriptionally regulate genes involved in heme homeostasis. Cellular requirements for heme are fulfilled by a cell’s internal capacity to synthesize its own heme in a cell-autonomous manner. However, growing evidence in vertebrates predicts that cellular heme levels in animals are not only maintained by heme synthesis, but also by distally located proteins that could signal systemic heme requirements to an inter-organ heme trafficking network through cell-nonautonomous regulation. Using C. elegans, a genetically and optically amenable animal model for visualizing heme-dependent signaling, we show that HRG-7, an aspartic protease homolog, mediates inter-organ signaling between the intestine and neuron. Loss of hrg-7 results in robust expression of intestinal heme importers and, remarkably, this occurs even under heme replete conditions when such transporters are not normally expressed. HRG-7 functions as a secreted signaling factor, independent of a functional enzymatic active site, and communicates intestinal heme status with extraintestinal tissues by integrating a DBL-1/BMP -dependent response from the neurons to transcriptionally regulate intestinal heme homeostasis. Given the evidence indicating that mechanisms of heme transport are conserved across metazoa, it is conceivable that the cell-nonautonomous signaling framework that we uncovered in C. elegans may have functional relevance for inter-organ regulation of iron and heme metabolism in humans. Overall design: Comparison of mRNA profiles from dbl-1(nk3) mutant C. elegans vs. wildtype (WT) broodmates and hrg-7(tm6801) mutants vs (WT) broomates fed OP50 E. coli or OP50 + 50µM heme. Biological duplicates were analyzed for dbl-1(nk3) mutants and (WT) broodmates. Biological triplicates were analyzed for hrg-7(tm6801) mutants and (WT) broodmates.

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Sinclair J, Pinter K, Samuel T, Beardsley S, Yuan X, Zhang J, Meng K, Yun S, Krause M, Hamza I