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

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.