At 23:00 on the evening of October 19, 2022, Beijing time, School of Life Sciences, Zhejiang University, Chen Caiyong Laboratory, published an article titled "HRG-9 homologues regulate haem trafficking from haem-enriched competitions" in Nature.
This study revealed a new intracellular heme chaperone HRG-9 (heme responsive gene-9). In C. nematode, HRG-9 and its homologous protein HRG-10 transport heme out of its storage site - lysosome-related organelles; and in organisms such as humans, zebrafish , yeast , and HRG-9's homologous protein TANGO2 transports heme out of its synthetic site - mitochondrial .
heme (heme) is a class of porphyrin compounds containing iron ion . In addition to being responsible for binding to oxygen in hemoglobin, it is also a cofactor of protein , cytochrome, cytochrome P450, catalase , peroxidase and other protein , and is involved in the regulation of gene expression , miRNA processing, circadian rhythm and other processes. The heme in cells comes from the synthesis site mitochondria or absorption site cell membrane , and the proteins that require heme are widely distributed in various subcellular sites. Free heme is hydrophobic and cytotoxic, so cells need to rely on specific transport proteins to transport and utilize heme [1,2].
To study the heme transport pathway, the researchers first carried out screening on the model animal Caenorhabditis elegans. C. nematode is a heme nutritionally deficient animal. It cannot synthesize heme itself, but it also has a variety of proteins such as cytochrome, cytochrome P450, catalase, etc. that require heme [3]. C. nematodes rely entirely on the transport system to take heme from food, so it is an ideal model for studying heme transport. The researchers cultured nematode with different concentrations of heme, and discovered a new heme sensor gene hrg-9 through transcriptomics analysis.
To explore the function of hrg-9, the researchers used heme-sensing nematodes (carrying hrg-1p::gfp) to conduct research and found that after deleting hrg-9 and/or its homologous gene hrg-10, nematodes exhibit an ischemic phenotype; and these knockout nematodes can tolerate the highly toxic heme homolog - protoporphy gallium. However, the absence of hrg-9 and hrg-10 does not affect the heme absorption capacity of nematodes and the total heme level of nematodes. Therefore, the researchers further used a fluorescent heme homolog, Meta-porphyrin zinc, to explore the distribution of heme in cells, and found that in the intestinal cells of hrg-9 and hrg-10 knockout nematode, a large amount of Meta-porphyrin zinc accumulates in the heme storage site, lysosome-related organelles. Fluorescence tracing experiments on this heme homolog showed that knocking out hrg-9 and hrg-10 affected the efflux of heme in lysosome-related organelles. These results suggest that C. nematode HRG-9 and HRG-10 are responsible for transporting heme out of storage sites for cell utilization.
Researchers found that mammals and other organisms with heme synthesis ability also have a homologous gene of hrg-9 - TANGO2 (transport and Golgi organization). It was originally believed to be involved in regulating protein transport and Golgi structure [4]. To determine whether TANGO2 also regulates heme homeostasis, the researchers used multiple heme sensing systems to study the TANGO2 gene in yeast, zebrafish and mammals. genetics experiments showed that TANGO2, which expresses yeast, zebrafish or humans, can all rescue heme transport defects caused by the loss of hrg-9/hrg-10, indicating that TANGO2 and HRG-9 have similar functions in heme homeostasis regulation. After yeast and mammalian cells, TANGO2 were deletion of TANGO2, heme accumulates in the mitochondria of the synthetic site. Through biochemical experiments, researchers found that TANGO2 protein can bind heme and transfer heme out of mitochondria.
HRG-9/TANGO2 does not contain transmembrane domains. How does it transport heme out of heme-rich organelles ? In vitro heme transport experiments show that TANGO2 can transport and bind heme on the mitochondrial membrane, and this transport function does not depend on other mitochondrial proteins. Based on the above results, HRG-9/TANGO2 is a functionally conserved heme chaperone protein, responsible for the mobilization and utilization of heme in cells. In organisms without heme synthesis ability, HRG-9 and its homologous protein HRG-10 transport heme from the storage site; and in organisms that can synthesize heme, TANGO2 transports heme from the synthetic site.
HRG-9 and its homologous protein transport heme
HRG-9 and its homologous proteins mutations in human TANGO2 gene will cause a rare hereditary disease [5, 6]. All the patients reported are children, and the patients have various symptoms such as developmental delay, rhabdomyolysis, arrhythmia, , epilepsy, metabolic syndrome, etc. At present, the cause of the disease caused by the TANGO2 mutation is still unclear. Researchers found that zebrafish's tango2 is also crucial for early growth and development. After knocking out tango2 on zebrafish, the young fish showed encephalopathy, arrhythmia , muscle damage, and died early in development. These phenotypes are similar to the clinical symptoms of sick children. Therefore, this mutant zebrafish provides a disease model for studying the causes and treatment strategies of TANGO2 disease.
This study discovered the intracellular heme molecular chaperones HRG-9 and TANGO2 for the first time, and revealed an important mechanism of heme transport and utilization in cells. At the same time, it clarified the biological function of TANGO2 in heme metabolism, providing a basis for exploring the pathological mechanism of TANGO2 disease.
Related paper information:
https://doi.org/10.1038/s41586-022-05347-z
references
1. Severance, S. Hamza, I. Trafficking of heme and porphyrins in metazoa. Chem. Rev. 109, 4596–4616 (2009).
2. Reddi, A. R. Hamza, I. Heme mobilization in animals: a metallolipid’s journey. Acc.Chem. Res. 49, 1104–1110 (2016).
3. Rao, A. U., Carta, L. K., Lesuisse, E. Hamza, I. Lack of heme synchronization in a free-living eukaryote. Proc. Natl Acad. Sci. USA 102, 4270–4275 (2005).
4. Bard, F. et al. Functional genomics reveals genes involved in protein secretion and Golgi organization. Nature 439, 604–607 (2006).
5. Kremer, L. S. et al. Bi-allelic truncating mutations in TANGO2 cause infancy-onset recurrent metabolic crises with encephalocardiomypathy. Am. J. Hum. Genet. 98, 358–362 (2016).
6. Lalani, S. R. et al. Recurrent muscle weakness with rhabdomyolysis, metabolic crises, and cardiac arrhythmia due to bi-allelic TANGO2 mutations. Am. J. Hum. Genet. 98, 347–357 (2016).
