Mechanisms of brain iron transport: insight into neurodegeneration and CNS disorders

Abstract

Trace metals such as iron, copper, zinc, manganese, and cobalt are essential cofactors for many cellular enzymes. Extensive research on iron, the most abundant transition metal in biology, has contributed to an increased understanding of the molecular machinery involved in maintaining its homeostasis in mammalian peripheral tissues. However, the cellular and intercellular iron transport mechanisms in the central nervous system (CNS) are still poorly understood. Accumulating evidence suggests that impaired iron metabolism is an initial cause of neurodegeneration, and several common genetic and sporadic neurodegenerative disorders have been proposed to be associated with dysregulated CNS iron homeostasis. This review aims to provide a summary of the molecular mechanisms of brain iron transport. Our discussion is focused on iron transport across endothelial cells of the blood-brain barrier and within the neuro- and glial-vascular units of the brain, with the aim of revealing novel therapeutic targets for neurodegenerative and CNS disorders.

Keywords: Blood-brain barrier (BBB); Reactive-Oxygen Species (ROS); brain vascular endothelial cell (BVEC); divalent metal transporter-1 (DMT1, Slc11a2); early endosome (EE); ferritin (Ft); ferroportin (Fpn); non-transferrin-bound iron (NTBI); transferrin (Tf); transient receptor potential mucolipin 1 (TRPML1).

Figure 1. Molecular mechanisms of intracellular iron transport

Most cellular uptake of ferric iron (Fe³⁺) occurs via receptor-mediated endocytosis of transferrin (Tf) (vesicular import pathway IN2). Fe³⁺ ions form complexes with the high affinity iron binding protein, Tf, which then binds to specific membrane-bound Tf receptor (TfR). After endocytosis, the acidic environment of the early endosomes (EE; compartment specificity defined by a small GTPase Rab5) triggers the release of Fe³⁺ from the Tf-TfR complex, which is recycled to the plasma membrane via Rab11-positive recycling endosomes (RE; vesicular export pathway EX2). Members of the Steap family of ferric reductases localize to the endosome and reduce Fe³⁺ (ferric) to its Fe²⁺ (ferrous) form before Fe²⁺ is released into the cytosol by the divalent metal transporter-1 (DMT1) in an H⁺-dependent manner. Fe³⁺ may also be sorted into Rab7-positive late endosomes (LE) and lysosomes (LY), where it is reduced into Fe²⁺ by Steap proteins in LE or LY. In LE and LY, Fe²⁺ can also be released by other endolysosomal iron release channels/transporters such as TRPML1, Nramp1 (in macrophages), or Zip8. Other sources of lysosomal Fe³⁺ include Fe³⁺-laden ferritin (Ft) complexes and autophagic ingestion of damaged organelles such as mitochondria. Other mechanisms for receptor-mediated iron uptake exist in oligodendrocytes and other cells, where Fe³⁺ ions bind the Ft receptors (Scara5 or Tim-2) and undergo receptor-mediated endocytosis (vesicular iron import pathway IN1). This pathway depends on lysosomal degradation of Ft-Fe complexes before iron is released into the cytosol. Lysosomal Fe³⁺ or Ft-Fe complexes may also be exported out of the cells via lysosomal exocytosis (vesicular export pathway EX1). In some cell types, uptake of non-Tf bound iron (NTBI) in its Fe²⁺ form can be mediated by plasma membrane localized iron importers such as DMT1 (non-vesicular import pathway IN3), TRPC6 (IN4), voltage-gated Ca²⁺ channels (VGCC; IN5), and Zip14 (Slc39a14; IN6). Free Fe²⁺ in the cytosol constitutes a “labile iron” pool (LIP; ~ 3 µM) for cellular utilization. If not immediately used, it can also be rapidly sequestered by cytosolic Ft into a non-reactive state. Finally, iron can be released from cells by the iron exporter ferroportin (Fpn). Cellular export of Fe²⁺ usually is coupled to a membrane bound or cytosolic ferroxidase such as Hephaestin (Hp) or Ceruloplasmin (Cp) that oxidizes reactive Fe²⁺ to its less reactive Fe³⁺ form before diffusing throughout the extracellular space. Fe³⁺ can be bound in the extracellular space by Tf, citrate, ascorbate, or ATP. Cytosolic or intralysosomal iron overload may catalyze the production of free radical oxides via the Fenton reaction. Radical oxides may cause cellular damage by oxidizing macromoleucles such as lipids, DNA, and proteins.

Figure 2. Iron transport across the blood-brain barrier (BBB)

Iron is sequestered in a non-reactive state in the blood plasma by transferrin (Tf; ~ 40 µM). Material transport from the blood plasma into the brain is limited by the blood-brain barrier, which is formed by tight junctions between the brain vascular endothelia cells (BVECs). Transferrin receptors (TfRs) line the lumen of the brain microvasculature and bind circulating Tf-Fe₂ (~ 10 µM) and facilitate iron uptake into BVECs via receptor-mediated endocytosis. Two models for BVEC iron export are shown. The DMT1/ferroportin (Fpn)-independent pathway releases Fe³⁺ or Tf-Fe₂ through the exocytosis of recycling endosomes (vesicular export pathway). Tf (mainly secreted from oligodendrocytes) concentration (<0.5 µM) is low in the CSF. Thus, the majority of Fe³⁺ forms complexes with ascorbate, citrate, and ATP as NTBI (~ 1 µM). Citrate and ATP are released from astrocytes. DMT1/Fpn-dependent (non-vesicular export) pathway releases Fe²⁺, which is rapidly converted into Fe³⁺ via ceruloplasmin (Cp) in the abluminal membranes. Alternatively, astrocytic foot-processes form close associations with BVECs, and polarized DMT1 expression in these astrocytic end-feet may facilitate rapid Fe²⁺ uptake after release (from BVECs) into the perivascular space of the brain. Neither astrocytes nor oligodendrocytes express detectable levels of TfR. Oligodendrocytes can take up iron via the ferritin (Ft) receptor Tim-2. In addition, oligodendrocytes may also uptake NTBI via DMT1 or other non-vesicular iron import mechanisms. The axons of neurons are wrapped with the myelin sheath (MS), which is made in oligodendrocytes in an iron-dependent manner.