Neuronal uptake of iron: somatopetal axonal transport and fate of cationized and native ferritin, and iron-dextran after intramuscular injections

Exploring Potential Mechanisms Accounting for Iron Accumulation in the Central Nervous System of Patients with Alzheimer's Disease

Elevated levels of iron occur in both cortical and subcortical regions of the CNS in patients with Alzheimer's disease. This accumulation is present early in the disease process as well as in more advanced stages. The factors potentially accounting for this increase are numerous, including: (1) Cells increase their uptake of iron and reduce their export of iron, as iron becomes sequestered (trapped within the lysosome, bound to amyloid β or tau, etc.); (2) metabolic disturbances, such as insulin resistance and mitochondrial dysfunction, disrupt cellular iron homeostasis; (3) inflammation, glutamate excitotoxicity, or other pathological disturbances (loss of neuronal interconnections, soluble amyloid β, etc.) trigger cells to acquire iron; and (4) following neurodegeneration, iron becomes trapped within microglia. Some of these mechanisms are also present in other neurological disorders and can also begin early in the disease course, indicating that iron accumulation is a relatively common event in neurological conditions. In response to pathogenic processes, the directed cellular efforts that contribute to iron buildup reflect the importance of correcting a functional iron deficiency to support essential biochemical processes. In other words, cells prioritize correcting an insufficiency of available iron while tolerating deposited iron. An analysis of the mechanisms accounting for iron accumulation in Alzheimer's disease, and in other relevant neurological conditions, is put forward.

Ferritin, a protein containing iron nanoparticles, induces reactive oxygen species formation and inhibits glutamate uptake in rat brain synaptosomes

Nanoparticles are currently used in medicine as agents for targeted drug delivery and imaging. However it has been demonstrated that nanoparticles induce neurodegeneration in vivo and kill neurons in vitro. The cellular and molecular bases of this phenomenon are still unclear. We have used the protein ferritin as a nanoparticle model. Ferritin contains iron particles (Fe(3+)) with size 7 nm and a protein shell. We investigated how ferritin influences uptake and release of [(14)C]glutamate and free radical formation as monitored by fluorescent dye DCFDA in rat brain synaptosomes. We found that even a high concentration of ferritin (800 microg/ml) did not induce spontaneous [(14)C]glutamate release. In contrast the same concentration of this protein inhibited [(14)C]glutamate uptake two fold. Furthermore ferritin induced intrasynaptosomal ROS (reactive oxygen species) formation in a dose-dependent manner. This process was insensitive to 30 microM DPI, an inhibitor of NADPH oxidase and to 10 microM CCCP, a mitochondrial uncoupler. These results indicate that iron-based nanoparticles can cause ROS and decreased glutamate uptake, potentially leading to neurodegeneration.

Nanotoxicology: an emerging discipline evolving from studies of ultrafine particles

Although humans have been exposed to airborne nanosized particles (NSPs; < 100 nm) throughout their evolutionary stages, such exposure has increased dramatically over the last century due to anthropogenic sources. The rapidly developing field of nanotechnology is likely to become yet another source through inhalation, ingestion, skin uptake, and injection of engineered nanomaterials. Information about safety and potential hazards is urgently needed. Results of older biokinetic studies with NSPs and newer epidemiologic and toxicologic studies with airborne ultrafine particles can be viewed as the basis for the expanding field of nanotoxicology, which can be defined as safety evaluation of engineered nanostructures and nanodevices. Collectively, some emerging concepts of nanotoxicology can be identified from the results of these studies. When inhaled, specific sizes of NSPs are efficiently deposited by diffusional mechanisms in all regions of the respiratory tract. The small size facilitates uptake into cells and transcytosis across epithelial and endothelial cells into the blood and lymph circulation to reach potentially sensitive target sites such as bone marrow, lymph nodes, spleen, and heart. Access to the central nervous system and ganglia via translocation along axons and dendrites of neurons has also been observed. NSPs penetrating the skin distribute via uptake into lymphatic channels. Endocytosis and biokinetics are largely dependent on NSP surface chemistry (coating) and in vivo surface modifications. The greater surface area per mass compared with larger-sized particles of the same chemistry renders NSPs more active biologically. This activity includes a potential for inflammatory and pro-oxidant, but also antioxidant, activity, which can explain early findings showing mixed results in terms of toxicity of NSPs to environmentally relevant species. Evidence of mitochondrial distribution and oxidative stress response after NSP endocytosis points to a need for basic research on their interactions with subcellular structures. Additional considerations for assessing safety of engineered NSPs include careful selections of appropriate and relevant doses/concentrations, the likelihood of increased effects in a compromised organism, and also the benefits of possible desirable effects. An interdisciplinary team approach (e.g., toxicology, materials science, medicine, molecular biology, and bioinformatics, to name a few) is mandatory for nanotoxicology research to arrive at an appropriate risk assessment.

Age-dependent uptake and retrograde axonal transport of exogenous albumin and transferrin in rat motor neurons

This study presents evidence for retrograde axonal transport of exogenous albumin and transferrin in adult brainstem motor neurons, whereas plasma proteins are not transported in neonatal motor neurons. The plasma protein uptake in motor neurons was dose-dependent, suggesting a nonspecific (fluid-phase) uptake mechanism. Further evidence for nonspecific uptake of exogenous transferrin in the motor neuron was found in the presence of transferrin receptor only on the soma and not on the axon terminal. The immunoreaction product of the exogenous plasma proteins was localized as perinuclear granules in association with the lysosomal system, as verified by staining for the lysosomal marker cathepsin D and by ultrastructural examinations. The results suggest that albumin and transferrin derived from hepatic synthesis gain access to motor neurons nonspecifically by retrograde axonal transport, whereas transferrin derived from intracerebral synthesis specifically gains access to motor neurons due to receptor-mediated uptake at the soma of the neuron. The lack of plasma proteins in developing motor neurons suggests that retrograde axonal transport of plasma proteins has no significance for developing axons. Plasma proteins have a potential for transporting toxic metals to motor neurons. Intraneuronal uptake of aluminum-transferrin either by nonspecific uptake in axon terminals or by receptor-mediated uptake at the soma may have a role in the pathogenesis of the motor neuron disease amyotrophic lateral sclerosis.

A review of axonal transport of metals

Neurons have efficient mechanisms for the transport of organelles and chemical substances in axons to the nerve terminals and back to the cell bodies. Enzymes involved in transmitter synthesis, peptide transmitters and their precursors are examples of macromolecules that are transported down the axon, anterogradely. For final degradation and possible reuse, many constituents are transported back to the cell body, retrogradely. Retrograde transport is also a pathway by which certain toxins may bypass the blood-brain barrier and accumulate in neurons. In recent years, it has been shown that certain metals may accumulate in neurons following retrograde transport. The metals for which retrograde transport has been demonstrated include lead, cadmium and mercury. In this article recent findings regarding axonal transport of metals are reviewed. The putative mechanisms involved in the uptake of metals into the nerve terminal and the fate of metals in the cell body are outlined. Axonal transport of metals as a possible etiological factor in diseases of the human nervous system is discussed.

Accumulation of mercury in brainstem nuclei of mice after retrograde axonal transport

Adult mice were injected intramuscularly in the region of the vibrissae muscles on the left side of the nose with a small volume of mercuric chloride dissolved in distilled water. The animals were killed after 1-6 weeks and fixed by whole-body perfusion. Frozen sections were taken from different levels of the brain stem and from the kidney. The sections were subjected to silver acetate autometallography for visualization of mercury. Mercury was found to accumulate in neurones of the facial nerve nuclei, of the motor trigeminal nuclei and of the trigeminal mesencephalic nuclei of the brain stem, after retrograde axonal transport. Mercury was also demonstrated in proximal tubular cells of the kidney. The mechanism for uptake of mercury at the neuromuscular junctions, and the fate of mercury within neurones are analysed. The possible significance of retrograde metal transport for the development of motor neurone disease is discussed.

Retrograde axonal transport of asialoglycoproteins in mouse trigeminal neurons in vivo and in rat dorsal root ganglia neurons in vitro

Different terminal sugars of the glycoprotein orosomucoid were exposed by sequential glycosidase digestions. The orosomucoid and its different derivatives were conjugated to horseradish peroxidase by a two-step glutaraldehyde coupling procedure, injected into the snout of 12-day-old mice or exposed to dorsal root ganglia neurons from embryonic rats, cultivated in a two chamber system. A marked increase in transport of the conjugates in the trigeminal and dorsal root ganglia neurons was observed histochemically after removal of sialic acid, exposing galactose as the terminal sugar. Quantitative hydrolysis of galactose residues resulted in reduced uptake. The data suggest the presence of a galactose-recognition molecule in the axon-terminal membrane, involved in retrograde axonal transport.

Inhibition of spontaneous transmitter release by the membrane ligand cationized ferritin

Exposure of the frog neuromuscular junction to cationized ferritin (200 micrograms ml-1) results in a reduction in MEPP frequency of approximately 25% within 10 min. This effect is not dependent on extracellular Ca2+, and identical results are obtained in salines buffered with EGTA. It is suggested that cationized ferritin does not inhibit transmitter release via a reduction in the intracellular Ca2+ concentration, but exerts its effect by a direct action on exocytosis. A mode of action is proposed. Unlike Con A, cationized ferritin is unable to suppress the stimulation of spontaneous release caused by high osmolarity.