Amyloids are a broad class of proteins and peptides that can misfold and assemble into long unbranched fibrils with a cross-β conformation. These misfolding and aggregation events are associated with the onset of a variety of human diseases, among them, Alzheimer's disease, Parkinson's disease, and Huntington disease. Our understanding of amyloids has been greatly supported by fluorescent molecular probes, such as thioflavin-T, which shows an increase in fluorescence emission upon binding to fibrillar aggregates. Since the first application of thioflavin-T in amyloid studies nearly 30 years ago, many probes have emerged exhibiting a variety of responses to amyloids, such as intensity changes, shifts in fluorescence maxima, and variations in lifetimes, among many others. These probes have shed light on a variety of topics including the kinetics of amyloid aggregation, the effectiveness of amyloid aggregation inhibitors, the elucidation of binding sites in amyloid structures, and the staining of amyloids aggregates in vitro, ex vivo, and in vivo. In this Review, we discuss the design, properties, and application of photoactive probes used to study amyloid aggregation, as well as the challenges faced by current probes and techniques, and the novel approaches that are emerging to address these challenges.

Interrogating Amyloid Aggregates using Fluorescent Probes.

Neurodegenerative diseases are debilitating disorders that feature progressive and selective loss of function or structure of anatomically or physiologically associated neuronal systems. Both chronic and acute neurodegenerative diseases are associated with high morbidity and mortality along with the death of neurons in different areas of the brain; moreover, there are few or no effective curative therapy options for treating these disorders. There is an urgent need to diagnose neurodegenerative disease as early as possible, and to distinguish between different disorders with overlapping symptoms that will help to decide the best clinical treatment. Recently, in neurodegenerative disease research, fluorescent-probe-mediated biomarker visualization techniques have been gaining increasing attention for the early diagnosis of neurodegenerative diseases. A survey of fluorescent probes for sensing and imaging biomarkers of neurodegenerative diseases is provided. These imaging probes are categorized based on the different potential biomarkers of various neurodegenerative diseases, and their advantages and disadvantages are discussed. Guides to develop new sensing strategies, recognition mechanisms, as well as the ideal features to further improve neurodegenerative disease fluorescence imaging are also explored.

Fluorescent Diagnostic Probes in Neurodegenerative Diseases

Biological applications of Ru(II) polypyridyl complexes

Metal-involved theranostics: An emerging strategy for fighting Alzheimer’s disease

Metal Complexes that Bind to the Amyloid-β Peptide of Relevance to Alzheimer's Disease

The complexes cis-[Ru(phen)2(Apy)2]2+, Apy = 4-aminopyridine and 3,4-aminopyridine, are stable in aqueous solution with strong visible absorption. They present emission in the visible region with long lifetime that accumulates in the cytoplasm of Neuro2A cell line without appreciable cytotoxicity. The complexes also serve as mixed-type reversible inhibitors of human AChE and BuChE with high active site contact. cis-[Ru(phen)2(3,4Apy)2]2+ competes efficiently with DMPO by the OH• radical. Luminescence using fluorescence lifetime imaging (FLIM) enables real-time imaging of the conformational changes of the self-aggregation of Aβ with incubation of complexes (0–24 h) in phosphate buffer at micromolar concentrations. By this technique, we identified protofibrills in the self-assembly of Aβ1–40 and globular structures in the short fragment Aβ15–21 in aqueous solution.

Luminescent Ru(II) Phenanthroline Complexes as a Probe for Real-Time Imaging of Aβ Self-Aggregation and Therapeutic Applications in Alzheimer’s Disease

Significance It has been recently shown that the speed of the conversion of α-synuclein from its native state into amyloid fibrils can be greatly enhanced by the formation of an intermediate dense liquid state formed through phase separation. The corresponding mechanism, however, remains to be fully characterized. In this work, we use a combination of fluorescence microscopy, microfluidics, and chemical kinetics to determine the microscopic rate constants for the primary nucleation and aggregate-dependent proliferation at neutral pH of α-synuclein fibrils. The approach that we report facilitates the study of the aggregation process of α-synuclein under physiological conditions and of its relationship with Parkinson’s disease and related synucleinopathies.

Spontaneous nucleation and fast aggregate-dependent proliferation of α-synuclein aggregates within liquid condensates at neutral pH

Comparison of Aβ (1-40, 1-28, 11-22, and 29-40) aggregation processes and inhibition of toxic species generated in early stages of aggregation by a water-soluble ruthenium complex

Mariana P. Cali

Papers

Luminescent Ru(II) Phenanthroline Complexes as a Probe for Real-Time Imaging of Aβ Self-Aggregation and Therapeutic Applications in Alzheimer’s Disease

Spontaneous nucleation and fast aggregate-dependent proliferation of α-synuclein aggregates within liquid condensates at neutral pH

Comparison of Aβ (1-40, 1-28, 11-22, and 29-40) aggregation processes and inhibition of toxic species generated in early stages of aggregation by a water-soluble ruthenium complex

Luminescent imaging of insulin amyloid aggregation using a sensitive ruthenium-based probe in the red region