One ring to bring them all and in the darkness bind them: The trafficking of heme without deliverers

Heme is essential for the survival of virtually all living systems—from bacteria, fungi, and yeast, through plants to animals. No eukaryote has been identified that can survive without heme. There are thousands of different proteins that require heme in order to function properly, and these are responsible for processes such as oxygen transport, electron transfer, oxidative stress response, respiration, and catalysis. Further to this, in the past few years, heme has been shown to have an important regulatory role in cells, in processes such as transcription, regulation of the circadian clock, and the gating of ion channels. To act in a regulatory capacity, heme needs to move from its place of synthesis (in mitochondria) to other locations in cells. But while there is detailed information on how the heme lifecycle begins (heme synthesis), and how it ends (heme degradation), what happens in between is largely a mystery. Here we summarize recent information on the quantification of heme in cells, and we present a discussion of a mechanistic framework that could meet the logistical challenge of heme distribution.

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“…The authors suggested direct membrane contacts between mitochondria and the ER as a possible pathway for heme delivery to the nucleus. 170 , 171 …”

Section: Quantifying Heme Concentrations In Cellsmentioning

confidence: 99%

Understanding the Logistics for the Distribution of Heme in Cells

Gallio

Fung

Cammack-Najera

et al. 2021

JACS Au

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“…The concentration of metabolites and lipids were then determined with mass spectrometry using MxP® Quant 500 kit (Biocrates Life Sciences AG, Innsbruck, Austria). The kit enables quantification of approximately 630 endogenous metabolites belonging to 26 different biochemical classes: alkaloids (1), amine oxides (1), amino acids (20), amino acids related compounds (30), bile acids (14), biogenic amines (9), carbohydrates (1), carboxylic acids (7), cresols (1), fatty acids (12), hormones (4), indoles and derivatives (4), nucleobases and related compounds (2), vitamins (1), acylcarnitines (40), lysophosphatidylcholines (14), phosphatidylcholines (76), sphingomyelins (15), ceramides (28), dihydroceramides (8), hexosylceramides (19), dihexosylceramides (9), trihexosylceramides (6), cholesteryl esters (22), diglycerides (44), triglycerides (242). The method combines direct infusion coupled with tandem mass spectrometry (DI -MS/MS) and reverse phase liquid chromatography coupled with tandem mass spectrometry workflows (LC -MS/MS).…”

Section: Targeted Metabolomics Of Subcellular Fractionation Of Testesmentioning

confidence: 99%

“…While cell biological studies in mouse embryonic fibroblasts from MRP5 KO mice confirmed that MRP5 indeed plays a conserved role in heme export i.e. transporting heme into the secretory pathway for incorporation into hemoproteins or exporting it from the cell across the plasma membrane (14, 18–20), MRP5 KO mice do not show any heme-related phenotypes in vivo . A postulated hypothesis to explain these differences could be genetic compensation by other closely related transporters, which are phylogenetically conserved in higher order eukaryotes but absent in the C. elegans genome (14, 21).…”

Section: Introductionmentioning

confidence: 99%

MRP5 and MRP9 Play a Concerted Role in Male Reproduction and Mitochondrial Function

Chambers

Kumar

Lichtenberg

et al. 2021

Preprint

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Multidrug Resistance Proteins (MRPs) are transporters that play critical roles in cancer even though the physiological substrates of these enigmatic transporters are poorly elucidated. In Caenorhabditis elegans, MRP5/ABCC5 is an essential heme exporter as mrp-5 mutants are unviable due to their inability to export heme from the intestine to extra-intestinal tissues. Heme supplementation restores viability of these mutants but fails to restore male reproductive deficits. Correspondingly, cell biological studies show that MRP5 regulates heme levels in the mammalian secretory pathway even though MRP5 knockout (KO) mice do not show reproductive phenotypes. The closest homolog of MRP5 is MRP9/ABCC12, which is absent in C. elegans raising the possibility that MRP9 may genetically compensate for MRP5. Here, we show that MRP5 and MRP9 double KO mice are viable but reveal significant male reproductive deficits. Although MRP9 is highly expressed in sperm, MRP9 KO mice show reproductive phenotypes only when MRP5 is absent. Both ABCC transporters localize to mitochondrial-associated membranes (MAMs), dynamic scaffolds that associate the mitochondria and endoplasmic reticulum. Consequently, DKO mice reveal abnormal sperm mitochondria with reduced mitochondrial membrane potential and fertilization rates. Metabolomics show striking differences in metabolite profiles in the DKO testes and RNA-seq show significant alterations in genes related to mitochondrial function and retinoic acid metabolism. Targeted functional metabolomics reveal lower retinoic acid levels in the DKO testes and higher levels of triglycerides in the mitochondria. These findings establish a model in which MRP5 and MRP9 play a concerted role in regulating male reproductive functions and mitochondrial sufficiency.

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“…All heme in the cell partitions between exchange inert high affinity hemoproteins, such as cytochromes and other heme enzymes, and certain exchange labile heme complexes that buffer free heme down to nanomolar concentrations (8,(12)(13)(14)63,68,69). The factors that buffer heme are poorly understood, but likely consist of a network of heme binding proteins, nucleic acids, and lipid membranes (8,(12)(13)(14)63,66,67,70,71). Labile heme (LH) may act as a reservoir for bioavailable heme that can readily exchange with and populate heme binding sites in heme dependent or regulated enzymes and proteins.…”

Section: Introductionmentioning

confidence: 99%

“…Total cellular heme in yeast and various non-erythroid human cell lines is on the order of 1-20 µM (62)(63)(64)(65)(66)(67). All heme in the cell partitions between exchange inert high affinity hemoproteins, such as cytochromes and other heme enzymes, and certain exchange labile heme complexes that buffer free heme down to nanomolar concentrations (8,(12)(13)(14)63,68,69). The factors that buffer heme are poorly understood, but likely consist of a network of heme binding proteins, nucleic acids, and lipid membranes (8,(12)(13)(14)63,66,67,70,71).…”

Section: Introductionmentioning

confidence: 99%

Heme oxygenase-2 (HO-2) binds and buffers labile heme, which is largely oxidized, in human embryonic kidney cells

Da¹,

Moore²,

Liu

et al. 2021

Preprint

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Heme oxygenases (HO) detoxify heme by oxidatively degrading it into carbon monoxide, iron, and biliverdin, which is reduced to bilirubin and excreted. Humans express two isoforms: inducible HO-1, which is up-regulated in response to various stressors, including excess heme, and constitutive HO-2. While much is known about the regulation and physiological function of HO-1, comparatively little is known about the role of HO-2 in regulating heme homeostasis. The biochemical necessity for expressing constitutive HO-2 is largely dependent on whether heme is sufficiently abundant and accessible as a substrate under conditions in which HO-1 is not induced. By measuring labile heme, total heme, and bilirubin in human embryonic kidney HEK293 cells with silenced or over-expressed HO-2, and various HO-2 mutant alleles, we found that endogenous heme is too limiting to support HO-2 catalyzed heme degradation. Rather, we discovered that a novel role for HO-2 is to bind and buffer labile heme. Taken together, in the absence of excess heme, we propose that HO-2 regulates heme homeostasis by acting as a heme buffering factor in control of heme bioavailability. When heme is in excess, HO-1 is induced and both HO-2 and HO-1 can provide protection from heme toxicity by enzymatically degrading it. Our results explain why catalytically inactive mutants of HO-2 are cytoprotective against oxidative stress. Moreover, the change in bioavailable heme due to HO-2 overexpression, which selectively binds ferric over ferrous heme, is consistent with the labile heme pool being oxidized, thereby providing new insights into heme trafficking and signaling.

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One ring to bring them all and in the darkness bind them: The trafficking of heme without deliverers

Cited by 60 publications

References 292 publications

Understanding the Logistics for the Distribution of Heme in Cells

Understanding the Logistics for the Distribution of Heme in Cells

MRP5 and MRP9 Play a Concerted Role in Male Reproduction and Mitochondrial Function

Heme oxygenase-2 (HO-2) binds and buffers labile heme, which is largely oxidized, in human embryonic kidney cells

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