Degradation Products of Amyloid Protein: Are They The Culprits?
Authors: Zaretsky Dmitry V. , Zaretskaia Maria 
Published: December 2020

Objectives:

Beta-amyloid (Aβ) peptides are most toxic to cells in oligomeric form. It is commonly accepted that
oligomers can form ion channels in cell membranes and allow calcium and other ions to enter cells. The activation of other
mechanisms, such as apoptosis or lipid peroxidation, aggravates the toxicity, but it itself can result from the same initial
point, that is, ion disturbance due to an increased permeability of membranes. However, experimental studies of membrane
channels created by Aβ are surprisingly limited. Methods: Here, we report a novel flow cytometry technique which can be
used to detect increased permeability of membranes to calcium induced by the exposure to amyloid peptides. Calcium entry
into the liposome is monitored using calcium-sensitive fluorescent probe. Undamaged lipid membranes are not permeable to
calcium. Liposomes that are prepared in a calcium-free medium become able to accumulate calcium in a calcium-containing
medium only after the formation of channels. Using this technique, we demonstrated that the addition of short amyloid
fragment Aβ, which is known for its extreme toxicity on cultured neurons, readily increased membrane permeability to
calcium. However, neither similarly sized peptide Ab22-35 nor full-length peptide Ab1-42 were producing channels. The
formation of channels was observed in the membranes made of phosphatidylserine, a negatively charged lipid, but not in
membranes made of the neutral phosphatidylcholine.

Methods:

Here, we report a novel flow cytometry technique which can be
used to detect increased permeability of membranes to calcium induced by the exposure to amyloid peptides. Calcium entry
into the liposome is monitored using calcium-sensitive fluorescent probe. Undamaged lipid membranes are not permeable to
calcium. Liposomes that are prepared in a calcium-free medium become able to accumulate calcium in a calcium-containing
medium only after the formation of channels. Using this technique, we demonstrated that the addition of short amyloid
fragment Aβ, which is known for its extreme toxicity on cultured neurons, readily increased membrane permeability to
calcium. However, neither similarly sized peptide Ab22-35 nor full-length peptide Ab1-42 were producing channels. The
formation of channels was observed in the membranes made of phosphatidylserine, a negatively charged lipid, but not in
membranes made of the neutral phosphatidylcholine.

Results:

In the Discussion section, we have analyzed several issues which could be critical for understanding the
pathogenesis of Alzheimer’s disease, specifically 1) the need for a negatively charged membrane to produce the ion channel;
2) the potential role of the aggregated form in cellular toxicity of Ab peptides; 3) channel-forming ability of multiple
degradation products of amyloid; 4) non-specificity of ion channels formed by amyloid peptides. Potential targets of
channel-forming oligomers appear to be intracellular and are organelles well-known for dysfunction in Alzheimer’s disease
(mitochondria and lysosomes). In fact, lysosomes can also be the producers of degraded amyloid. Provided speculations
support the hypothesis that neuronal toxicity can be caused by the degradation products of beta-amyloid.