Copper(II) sequentially loads onto the N-terminal amino group of the cellular prion protein before the individual octarepeats

Are fibrinaloid microclots a cause of autoimmunity in Long Covid and other post-infection diseases?

It is now well established that the blood-clotting protein fibrinogen can polymerise into an anomalous form of fibrin that is amyloid in character; the resultant clots and microclots entrap many other molecules, stain with fluorogenic amyloid stains, are rather resistant to fibrinolysis, can block up microcapillaries, are implicated in a variety of diseases including Long COVID, and have been referred to as fibrinaloids. A necessary corollary of this anomalous polymerisation is the generation of novel epitopes in proteins that would normally be seen as 'self', and otherwise immunologically silent. The precise conformation of the resulting fibrinaloid clots (that, as with prions and classical amyloid proteins, can adopt multiple, stable conformations) must depend on the existing small molecules and metal ions that the fibrinogen may (and is some cases is known to) have bound before polymerisation. Any such novel epitopes, however, are likely to lead to the generation of autoantibodies. A convergent phenomenology, including distinct conformations and seeding of the anomalous form for initiation and propagation, is emerging to link knowledge in prions, prionoids, amyloids and now fibrinaloids. We here summarise the evidence for the above reasoning, which has substantial implications for our understanding of the genesis of autoimmunity (and the possible prevention thereof) based on the primary process of fibrinaloid formation.

Interaction between Hemin and Prion Peptides: Binding, Oxidative Reactivity and Aggregation

We investigate the interaction of hemin with four fragments of prion protein (PrP) containing from one to four histidines (PrP_106-114, PrP_95-114, PrP_84-114, PrP_76-114) for its potential relevance to prion diseases and possibly traumatic brain injury. The binding properties of hemin-PrP complexes have been evaluated by UV-visible spectrophotometric titration. PrP peptides form a 1:1 adduct with hemin with affinity that increases with the number of histidines and length of the peptide; the following log K₁ binding constants have been calculated: 6.48 for PrP_76-114, 6.1 for PrP_84-114, 4.80 for PrP_95-114, whereas for PrP_106-114, the interaction is too weak to allow a reliable binding constant calculation. These constants are similar to that of amyloid-β (Aβ) for hemin, and similarly to hemin-Aβ, PrP peptides tend to form a six-coordinated low-spin complex. However, the concomitant aggregation of PrP induced by hemin prevents calculation of the K₂ binding constant. The turbidimetry analysis of [hemin-PrP_76-114] shows that, once aggregated, this complex is scarcely soluble and undergoes precipitation. Finally, a detailed study of the peroxidase-like activity of [hemin-(PrP)] shows a moderate increase of the reactivity with respect to free hemin, but considering the activity over long time, as for neurodegenerative pathologies, it might contribute to neuronal oxidative stress.

Copper signalling: causes and consequences

Copper-containing enzymes perform fundamental functions by activating dioxygen (O2) and therefore allowing chemical energy-transfer for aerobic metabolism. The copper-dependence of O2 transport, metabolism and production of signalling molecules are supported by molecular systems that regulate and preserve tightly-bound static and weakly-bound dynamic cellular copper pools. Disruption of the reducing intracellular environment, characterized by glutathione shortage and ambient Cu(II) abundance drives oxidative stress and interferes with the bidirectional, copper-dependent communication between neurons and astrocytes, eventually leading to various brain disease forms. A deeper understanding of of the regulatory effects of copper on neuro-glia coupling via polyamine metabolism may reveal novel copper signalling functions and new directions for therapeutic intervention in brain disorders associated with aberrant copper metabolism.

Prion Peptides Are Extremely Sensitive to Copper Induced Oxidative Stress

Copper(II) binding to prion peptides does not prevent Cu redox cycling and formation of reactive oxygen species (ROS) in the presence of reducing agents. The toxic effects of these species are exacerbated in the presence of catecholamines, indicating that dysfunction of catecholamine vesicular sequestration or recovery after synaptic release is a dangerous amplifier of Cu induced oxidative stress. Cu bound to prion peptides including the high affinity site involving histidines adjacent to the octarepeats exhibits marked catalytic activity toward dopamine and 4-methylcatechol. The resulting quinone oxidation products undergo parallel oligomerization and endogenous peptide modification yielding catechol adducts at the histidine binding ligands. These modifications add to the more common oxidation of Met and His residues produced by ROS. Derivatization of Cu-prion peptides is much faster than that undergone by Cu-β-amyloid and Cu-α-synuclein complexes in the same conditions.

Interaction between Prion Protein's Copper-Bound Octarepeat Domain and a Charged C-Terminal Pocket Suggests a Mechanism for N-Terminal Regulation

Copper plays a critical role in prion protein (PrP) physiology. Cu(2+) binds with high affinity to the PrP N-terminal octarepeat (OR) domain, and intracellular copper promotes PrP expression. The molecular details of copper coordination within the OR are now well characterized. Here we examine how Cu(2+) influences the interaction between the PrP N-terminal domain and the C-terminal globular domain. Using nuclear magnetic resonance and copper-nitroxide pulsed double electron-electron resonance, with molecular dynamics refinement, we localize the position of Cu(2+) in its high-affinity OR-bound state. Our results reveal an interdomain cis interaction that is stabilized by a conserved, negatively charged pocket of the globular domain. Interestingly, this interaction surface overlaps an epitope recognized by the POM1 antibody, the binding of which drives rapid cerebellar degeneration mediated by the PrP N terminus. The resulting structure suggests that the globular domain regulates the N-terminal domain by binding the Cu(2+)-occupied OR within a complementary pocket.

Endocytosis of the tachykinin neuropeptide, neurokinin B, in astrocytes and its role in cellular copper uptake

The tachykinin neuropeptide, neurokinin B (NKB), belongs to a family of peptides having diverse roles in the brain. NKB, along with several other tachykinins, has been identified as a copper-binding peptide, however the physiological relevance of the binding is unclear. Previously, NKB was shown to limit the ability of copper to enter astrocytes and disrupt calcium homeostasis and it was thought that the peptide was sequestering the metal extracellularly. Here we use a fluorescein-labelled NKB peptide (F-NKB) to show that NKB is not retained extracellularly, but is endocytosed within 10-20min after addition to the cell media. The endocytosis is not inhibited when NKB is delivered as a copper-complex, [Cu^II(F-NKB)₂]. Endocytosis of NKB can increase intracellular copper. Comparison to cells cultured in copper-free buffer indicated that apo-NKB can facilitate uptake of copper found in normal culture media. To achieve this NKB must compete with a variety of copper proteins, and we show that NKB can successfully compete with copper-binding peptides derived from the prion protein, itself associated with Cu(II) and Zn(II) metabolism. We suggest a mechanism of receptor mediated endocytosis to account for the observations.

Truncated Amyloid-β(11-40/42) from Alzheimer Disease Binds Cu2+ with a Femtomolar Affinity and Influences Fiber Assembly

Alzheimer disease coincides with the formation of extracellular amyloid plaques composed of the amyloid-β (Aβ) peptide. Aβ is typically 40 residues long (Aβ(1-40)) but can have variable C and N termini. Naturally occurring N-terminally truncated Aβ(11-40/42) is found in the cerebrospinal fluid and has a similar abundance to Aβ(1-42), constituting one-fifth of the plaque load. Based on its specific N-terminal sequence we hypothesized that truncated Aβ(11-40/42) would have an elevated affinity for Cu(2+). Various spectroscopic techniques, complemented with transmission electron microscopy, were used to determine the properties of the Cu(2+)-Aβ(11-40/42) interaction and how Cu(2+) influences amyloid fiber formation. We show that Cu(2+)-Aβ(11-40) forms a tetragonal complex with a 34 ± 5 fm dissociation constant at pH 7.4. This affinity is 3 orders of magnitude tighter than Cu(2+) binding to Aβ(1-40/42) and more than an order of magnitude tighter than that of serum albumin, the extracellular Cu(2+) transport protein. Furthermore, Aβ(11-40/42) forms fibers twice as fast as Aβ(1-40) with a very different morphology, forming bundles of very short amyloid rods. Substoichiometric Cu(2+) drastically perturbs Aβ(11-40/42) assembly, stabilizing much longer fibers. The very tight fm affinity of Cu(2+) for Aβ(11-40/42) explains the high levels of Cu(2+) observed in Alzheimer disease plaques.

Cellular prion protein and NMDA receptor modulation: protecting against excitotoxicity

Although it is well established that misfolding of the cellular prion protein (PrP^C) into the β-sheet-rich, aggregated scrapie conformation (PrP^Sc) causes a variety of transmissible spongiform encephalopathies (TSEs), the physiological roles of PrP^C are still incompletely understood. There is accumulating evidence describing the roles of PrP^C in neurodegeneration and neuroinflammation. Recently, we identified a functional regulation of NMDA receptors by PrP^C that involves formation of a physical protein complex between these proteins. Excessive NMDA receptor activity during conditions such as ischemia mediates enhanced Ca²⁺ entry into cells and contributes to excitotoxic neuronal death. In addition, NMDA receptors and/or PrP^C play critical roles in neuroinflammation and glial cell toxicity. Inhibition of NMDA receptor activity protects against PrP^Sc-induced neuronal death. Moreover, in mice lacking PrP^C, infarct size is increased after focal cerebral ischemia, and absence of PrP^C increases susceptibility of neurons to NMDA receptor-dependent death. Recently, PrP^C was found to be a receptor for oligomeric beta-amyloid (Aβ) peptides, suggesting a role for PrP^C in Alzheimer's disease (AD). Our recent findings suggest that Aβ peptides enhance NMDA receptor current by perturbing the normal copper- and PrP^C-dependent regulation of these receptors. Here, we review evidence highlighting a role for PrP^C in preventing NMDA receptor-mediated excitotoxicity and inflammation. There is a need for more detailed molecular characterization of PrP^C-mediated regulation of NMDA receptors, such as determining which NMDA receptor subunits mediate pathogenic effects upon loss of PrP^C-mediated regulation and identifying PrP^C binding site(s) on the receptor. This knowledge will allow development of novel therapeutic interventions for not only TSEs, but also for AD and other neurodegenerative disorders involving dysfunction of PrP^C.