Heme Gazing: Illuminating Eukaryotic Heme Trafficking, Dynamics, and Signaling with Fluorescent Heme Sensors

Abstract

Heme (iron protoporphyrin IX) is an essential protein prosthetic group and signaling molecule required for most life on Earth. All heme-dependent processes require the dynamic and rapid mobilization of heme from sites of synthesis or uptake to hemoproteins present in virtually every subcellular compartment. The cytotoxicity and hydrophobicity of heme necessitate that heme mobilization be carefully controlled to mitigate the deleterious effects of this essential toxin. Indeed, a number of disorders, including certain cancers, cardiovascular diseases, and aging and age-related neurodegenerative diseases, are tied to defects in heme homeostasis. However, the molecules and mechanisms that mediate heme transport and trafficking, and the dynamics of these processes, are poorly understood. This is in large part due to the lack of physical tools for probing cellular heme. Herein, we discuss the recent development of fluorescent probes that can monitor and image kinetically labile heme with respect to its mobilization and role in signaling. In particular, we will highlight how heme gazing with these tools can uncover new heme trafficking factors upon being integrated with genetic screens and illuminate the concentration, subcellular distribution, and dynamics of labile heme in various physiological contexts. Altogether, the monitoring of labile heme, along with recent biochemical and cell biological studies demonstrating the reversible regulation of certain cellular processes by heme, is challenging us to reconceptualize heme from being a static cofactor buried in protein active sites to a dynamic and mobile signaling molecule.

Human Ribosomal G-Quadruplexes Regulate Heme Bioavailability

“…Taken together, the results here suggest that structural features of the human ribosome coupled with its high cytosolic abundance facilitate association with endogenous hemin. These results provide potentially new insights into the molecules and mechanisms underlying intracellular heme trafficking and bioavailability, which are currently poorly understood (1)(2)(3)39). Our results suggest that ribosomes, and G4 containing rRNA in particular, are central hubs of heme metabolism, acting to buffer intracellular heme and possibly regulate heme trafficking and cotranslational hemylation.…”

“…Cells tightly control heme concentration and bioavailability (1-3) because it is essential but potentially cytotoxic. Proteins that regulate heme concentration are relatively well understood; structures and mechanisms of all eight heme biosynthetic enzymes and the heme degrading heme oxygenases are known (1)(2)(3). However, regulation of heme bioavailability, including intracellular trafficking from sites of synthesis in the mitochondrial matrix or uptake at the plasma membrane, is poorly understood.…”

Mitochondrial-nuclear heme trafficking is regulated by GTPases that control mitochondrial dynamics

“…Heme (iron protoporphyrin IX) is an essential but inherently cytotoxic metallocofactor and signaling molecule 1,2 . As a cofactor, heme facilitates diverse processes that span electron transfer, chemical catalysis, and gas synthesis, storage and transport 1,2 .…”

“…Heme (iron protoporphyrin IX) is an essential but inherently cytotoxic metallocofactor and signaling molecule 1,2 . As a cofactor, heme facilitates diverse processes that span electron transfer, chemical catalysis, and gas synthesis, storage and transport 1,2 . As a signaling molecule, heme regulation of an array of proteins, including transcription factors 3–6 , kinases 7,8 , ion channels 9 , micro RNA processing proteins 10 , collectively control pathways spanning iron homeostasis, oxygen sensing, the oxidative stress response, mitochondrial respiration and biogenesis, mitophagy, apoptosis, circadian rhythms, cell cycle progression, proliferation, and protein translation and degradation 1,2 .…”

“…All heme dependent processes require that heme is mobilized from its site of synthesis on the matrix side of the mitochondrial inner membrane (IM) to heme proteins present in virtually every subcellular compartment 1,2 . However, the specific factors that govern the mobilization of heme out of the mitochondria, heme distribution to a multitude of locales, and heme insertion into target hemoproteins are not well-understood 1,2 . Our approach to better understand heme mobilization and utilization is to directly probe bioavailable or kinetically labile heme (LH) relevant for heme trafficking and signaling with fluorescent heme imaging agents 19 .…”