Cryo-EM structures and functional characterization of homo- and heteropolymers of human ferritin variants

Abstract

The role of abnormal brain iron metabolism in neurodegenerative diseases is still insufficiently understood. Here, we investigate the molecular basis of the neurodegenerative disease hereditary ferritinopathy (HF), in which dysregulation of brain iron homeostasis is the primary cause of neurodegeneration. We mutagenized ferritin’s three-fold pores (3FPs), i.e. the main entry route for iron, to investigate ferritin’s iron management when iron must traverse the protein shell through the disrupted four-fold pores (4FPs) generated by mutations in the ferritin light chain (FtL) gene in HF. We assessed the structure and properties of ferritins using cryo-electron microscopy and a range of functional analyses in vitro. Loss of 3FP function did not alter ferritin structure but led to a decrease in protein solubility and iron storage. Abnormal 4FPs acted as alternate routes for iron entry and exit in the absence of functional 3FPs, further reducing ferritin iron-storage capacity. Importantly, even a small number of MtFtL subunits significantly compromises ferritin solubility and function, providing a rationale for the presence of ferritin aggregates in cell types expressing different levels of FtLs in patients with HF. These findings led us to discuss whether modifying pores could be used as a pharmacological target in HF.

Hyperferritinemia: the link between COVID-19, inflammation, and patient comorbidities

“…Invertebrates, two types of ferritin subunits assemble to form the ferritin molecule: L (light) and H (heavy) ferritins, genetically encoded on the 11q and 19q chromosomes in humans. The ratio of H/L-ferritin subunits is variable depending on the tissue, type of cell, cell subcompartment, metabolic rate, environmental factors, growth factors, translational and post-translational changes, and the presence of a large array of diseases [3,4,6,8,10,13,14]. The H-subunit has ferroxidase activity and plays the key role of oxidizing ferrous ions (Fe2+) to ferric ions (Fe3+) that can be further be stored in a mineralized, stable internal core of ferritin [6,13].…”

“…The ratio of H/L-ferritin subunits is variable depending on the tissue, type of cell, cell subcompartment, metabolic rate, environmental factors, growth factors, translational and post-translational changes, and the presence of a large array of diseases [3,4,6,8,10,13,14]. The H-subunit has ferroxidase activity and plays the key role of oxidizing ferrous ions (Fe2+) to ferric ions (Fe3+) that can be further be stored in a mineralized, stable internal core of ferritin [6,13]. Out of the multiple oxidation iron states (from -2 to +6), Fe3+ is the most stable, non-toxic iron state [7,15].…”

“…The process of iron oxidation from Fe2+ to Fe3+ is therefore essential for the conversion of iron to a non-toxic form that can be stored inside of the cell; it is also important as it consumes free oxygen radicals in the process of iron oxidation that would otherwise accumulate and lead to cell damage [3,8]. L-ferritin subunit lacks a ferroxidase activity; it has a salt bridge in its folds and therefore plays a role only in the stability of the ferritin cage and in the ferric iron storage [1,13]. As the two subtypes of ferritin units have functional and structural differences, the ratio between the two in the ferritin nanocage differs between tissues, depending on the metabolic rate and need for a fast release of iron for incorporation into proteins.…”

“…Serum ferritin and intracellular ferritin have markedly increased levels in many inflammatory autoimmune diseases and acute phase reactions [10,19,20]. Such diseases that are characterized by chronic or acute inflammation, altered iron metabolism, and associate high serum ferritin levels include infections (acute, chronic, sepsis; viral, such as influenza infection, HCV and HIV; bacterial (including tuberculosis); autoimmune diseases, such as rheumatoid arthritis, systemic erythematosus lupus; kidney diseases; cancer (breast, colorectal, non-small lung cancer, prostate, pancreatic, oral, ovarian, renal); metabolic diseases (diabetes mellitus, obesity, dyslipidemia); cardiovascular diseases including hypertension; neurodegenerative diseases, such as Parkinson, Alzheimer, multiple sclerosis [1,2,4,10,13,16,[21][22][23][24][25][26][27][28][29][30][31][32][33][34][35]. The inflammatory state/chronic inflammatory microenvironment is considered a consequence, a trigger, and/or an aggravating factor for the diseases mentioned above, leading to increased patient morbidity and mortality rates and the development of other severe chronic diseases [30 36-39].…”

Heterozygous Nonsense Variants in the Ferritin Heavy Chain GeneFTH1Cause a Novel Pediatric Neuroferritinopathy

“…Neuropathology shows neuronal ferritin aggregates and neuroimaging indicates accumulation in the basal ganglia. Previous studies suggest pathogenic variants associated with FTL hereditary neuroferritinopathy map to the C-terminus of the L-ferritin subunit, disrupting the E-helix domain and affecting iron permeability and storage capacity [19][20][21] .…”