Copper binding and protein aggregation: a journey from the brain to the human lens.

Metal ions have been implicated in several proteinopathies associated to degenerative and neurodegenerative diseases. While the molecular mechanisms for protein aggregation are still under investigation, recent findings from Cryo-EM point out to polymorphisms in aggregates obtained from patients, as compared to those formed in vitro, suggesting that several factors may impact aggregation in vivo. One of these factors could be the direct binding of metal ions to the proteins engaged in aggregate formation. In this opinion article, three case studies are discussed to address the question of how metal ion binding to a peptide or protein may impact its conformation, folding, and aggregation, and how this may be relevant in understanding the polymorphic nature of the aggregates related to disease. Specifically, the impact of Cu2+ ions in the amyloid aggregation of amyloid-ß and amylin (or IAPP- islet amyloid polypeptide) are discussed and then contrasted to the case of Cu2+-induced non-amyloid aggregation of human lens γ-crystallin proteins. For the intrinsically disordered peptides amyloid-ß and IAPP, the impact of Cu2+ ion binding is highly dependent on the relative location of the metal binding site and the hydrophobic regions involved in ß-sheet folding and amyloid formation. Further structural studies of how Cu2+ binding impacts amyloid aggregation pathways and the molecular structure of the final amyloid fibril, both, in vitro and in vivo, will certainly shed light into the molecular origins of the polymorphisms observed in diseased tissue. Finally, contrasting these cases to that of Cu2+-induced non-amyloid aggregation of γ-crystallins, it is evident that, although the impact in aggregation - and the nature of the aggregate - may differ in each system, at the molecular level there is a competition between metal ion coordination and the stability of ß-sheet structures. Considering the importance of the ß-sheet fold in biology, it is fundamental to understand the energetics and molecular details behind such competition. This opinion article aims to highlight future research directions in the field that can help tackle the important question of how metal ion binding may impact protein folding and aggregation and how this relates to disease.

Dissecting the copper bioinorganic chemistry of the functional and pathological roles of the prion protein: Relevance in Alzheimer's disease and cancer.

The cellular prion protein (PrPC) is a metal-binding biomolecule that can interact with different protein partners involved in pivotal physiological processes, such as neurogenesis and neuronal plasticity. Recent studies profile copper and PrPC as important players in the pathological mechanisms of Alzheimer's disease and cancer. Although the copper-PrPC interaction has been characterized extensively, the role of the metal ion in the physiological and pathological roles of PrPC has been barely explored. In this article, we discuss how copper binding and proteolytic processing may impact the ability of PrPC to recruit protein partners for its functional roles. The importance to dissect the role of copper-PrPC interactions in health and disease is also underscored.

Amyloid ß Perturbs Cu(II) Binding to the Prion Protein in a Site-Specific Manner: Insights into Its Potential Neurotoxic Mechanisms.

Amyloid ß (Aß) is a Cu-binding peptide that plays a key role in the pathology of Alzheimer's disease. A recent report demonstrated that Aß disrupts the Cu-dependent interaction between cellular prion protein (PrPC) and N-methyl-d-aspartate receptor (NMDAR), inducing overactivation of NMDAR and neurotoxicity. In this context, it has been proposed that Aß competes for Cu with PrPC; however, there is no spectroscopic evidence to support this hypothesis. Prion protein (PrP) can bind up to six Cu(II) ions: from one to four at the octarepeat (OR) region, producing low- and high-occupancy modes, and two at the His96 and His111 sites. Additionally, PrPC is cleaved by α-secretases at Lys110/His111, yielding a new Cu(II)-binding site at the α-cleaved His111. In this study, the competition for Cu(II) between Aß(1-16) and peptide models for each Cu-binding site of PrP was evaluated using circular dichroism and electron paramagnetic resonance. Our results show that the impact of Aß(1-16) on Cu(II) coordination to PrP is highly site-specific: Aß(1-16) cannot effectively compete with the low-occupancy mode at the OR region, whereas it partially removes the metal ion from the high-occupancy modes and forms a ternary OR-Cu(II)-Aß(1-16) complex. In contrast, Aß(1-16) removes all Cu(II) ions from the His96 and His111 sites without formation of ternary species. Finally, at the α-cleaved His111 site, Aß(1-16) yields at least two different ternary complexes depending on the ratio of PrP/Cu(II)/Aß. Altogether, our spectroscopic results indicate that only the low-occupancy mode at the OR region resists the effect of Aß, while Cu(II) coordination to the high-occupancy modes and all other tested sites of PrP is perturbed, by either removal of the metal ion or formation of ternary complexes. These results provide important insights into the intricate effect of Aß on Cu(II) binding to PrP and the potential neurotoxic mechanisms through which Aß might affect Cu-dependent functions of PrPC, such as NMDAR modulation.

Neurotoxicity Linked to Dysfunctional Metal Ion Homeostasis and Xenobiotic Metal Exposure: Redox Signaling and Oxidative Stress.

SIGNIFICANCE: Essential metals such as copper, iron, manganese, and zinc play a role as cofactors in the activity of a wide range of processes involved in cellular homeostasis and survival, as well as during organ and tissue development. Throughout our life span, humans are also exposed to xenobiotic metals from natural and anthropogenic sources, including aluminum, arsenic, cadmium, lead, and mercury. It is well recognized that alterations in the homeostasis of essential metals and an increased environmental/occupational exposure to xenobiotic metals are linked to several neurological disorders, including neurodegeneration and neurodevelopmental alterations. Recent Advances: The redox activity of essential metals is key for neuronal homeostasis and brain function. Alterations in redox homeostasis and signaling are central to the pathological consequences of dysfunctional metal ion homeostasis and increased exposure to xenobiotic metals. Both redox-active and redox-inactive metals trigger oxidative stress and damage in the central nervous system, and the exact mechanisms involved are starting to become delineated. CRITICAL ISSUES: In this review, we aim to appraise the role of essential metals in determining the redox balance in the brain and the mechanisms by which alterations in the homeostasis of essential metals and exposure to xenobiotic metals disturb the cellular redox balance and signaling. We focus on recent literature regarding their transport, metabolism, and mechanisms of toxicity in neural systems. FUTURE DIRECTIONS: Delineating the specific mechanisms by which metals alter redox homeostasis is key to understand the pathological processes that convey chronic neuronal dysfunction in neurodegenerative and neurodevelopmental disorders. Antioxid. Redox Signal. 28, 1669-1703.

Strategies that Target Tight Junctions for Enhanced Drug Delivery.

Ions and molecules move across epithelial barriers by two pathways, the transcellular and the paracellular. The former is taken by lipophilic compounds, or by ions and molecules that move across the plasma membrane through pumps, carriers or exchangers. The second route is regulated by the tight junction (TJ) that through paracellular channels, allows the transport of ions across epithelial barriers. Since, a wide variety of bioactive molecules like peptides, proteins and oligonucleotides cannot use the transcellular route, due to their hydrophilic nature, interest has arisen in devising procedures to open the TJ in a reversible manner for paracellular drug delivery. Here, we describe how different strategies have been devised to enhance the paracellular intestinal absorption of drugs; to open the blood-brain barrier (BBB) to allow the penetration of drugs for the treatment of disorders and tumors of the central nervous system; or to deliver antigens into the subjacent mucosa associated lymphoid tissues, for the development of mucosal vaccines. The strategies described, include the use of peptides, antibodies and miRNAs that target proteins of the apical junctional complex, as well as toxins derived from microorganisms that open the TJ by inducing the contraction of the cortical actomyosin ring. Also, we describe how paracellular absorption, is enhanced by drugs that extract cholesterol from the plasma membrane, surfactants, fatty acids, oligosaccharides, cationic polymers, nitric oxide donors and calcium chelators. Likewise, we explain how the BBB has been opened by employing tumor necrosis factor-α, bradykinin, short chain alkylglycerols, hyperosmotic mannitol and focused ultrasound.