Researchers have found that excess levels of calcium in brain cells may lead to the formation of toxic clusters that are the hallmark of Parkinson's disease.
The international team found that calcium can mediate the interaction between small membranous structures inside nerve endings, which are important for neuronal signaling in the brain, and alpha-synuclein, the protein associated with Parkinson's disease. Excess levels of either calcium or alpha-synuclein may be what starts the chain reaction that leads to the death of brain cells.
The findings, reported in the journal Nature Communications, represent another step towards understanding how and why people develop Parkinson's. Parkinson's disease is one of a number of neurodegenerative diseases caused when naturally occurring proteins fold into the wrong shape and stick together with other proteins, eventually forming thin filament-like structures called amyloid fibrils. These amyloid deposits of aggregated alpha-synuclein, also known as Lewy bodies, are the sign of Parkinson's disease.
Curiously, it hasn't been clear until now what alpha-synuclein actually does in the cell: why it's there and what it's meant to do. It is implicated in various processes, such as the smooth flow of chemical signals in the brain and the movement of molecules in and out of nerve endings, but exactly how it behaves is unclear.
"Alpha-synuclein is a very small protein with very little structure, and it needs to interact with other proteins or structures in order to become functional, which has made it difficult to study," said senior author.
Thanks to super-resolution microscopy techniques, it is now possible to look inside cells to observe the behavior of alpha-synuclein. To do so, researchers isolated synaptic vesicles, part of the nerve cells that store the neurotransmitters which send signals from one nerve cell to another.
In neurons, calcium plays a role in the release of neurotransmitters. The researchers observed that when calcium levels in the nerve cell increase, such as upon neuronal signalling, the alpha-synuclein binds to synaptic vesicles at multiple points causing the vesicles to come together. This may indicate that the normal role of alpha-synuclein is to help the chemical transmission of information across nerve cells.
Authors show that calcium binds to the C terminus of alpha-synuclein, therewith increasing its lipid-binding capacity. They reveal that alpha-synuclein interacts with isolated synaptic vesicles with two regions, the N terminus, already known from studies on SUVs, and additionally via its C terminus, which is regulated by the binding of calcium. Their work with synaptosomes shows that calcium mediates the localization of alpha-synuclein at the pre-synaptic terminal, and an imbalance in calcium or alpha-synuclein can cause synaptic vesicle clustering, as seen ex vivo and in vitro.
"This is the first time we've seen that calcium influences the way alpha-synuclein interacts with synaptic vesicles," said the paper's first author. "We think that alpha-synuclein is almost like a calcium sensor. In the presence of calcium, it changes its structure and how it interacts with its environment, which is likely very important for its normal function."
"There is a fine balance of calcium and alpha-synuclein in the cell, and when there is too much of one or the other, the balance is tipped and aggregation begins, leading to Parkinson's disease," said co-first author.
The imbalance can be caused by a genetic doubling of the amount of alpha-synuclein (gene duplication), by an age-related slowing of the breakdown of excess protein, by an increased level of calcium in neurons that are sensitive to Parkinson's, or an associated lack of calcium buffering capacity in these neurons.
Understanding the role of alpha-synuclein in physiological or pathological processes may aid in the development of new treatments for Parkinson's disease. One possibility is that drug candidates developed to block calcium, for use in heart disease for instance, might also have potential against Parkinson's disease.
Calcium-synuclein interaction and neurodegenerative diseases
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