Study shows Alzheimer's disease-related peptides form toxic calcium channels in the plasma membrane

October 24, 2011

Alzheimer's disease is triggered by the inappropriate processing of amyloid precursor protein to generate excess amounts of short peptide fragments called A-beta. For many years, the neurodegeneration associated with Alzheimer's disease was thought to be caused by the buildup of A-beta in insoluble, fibrous plaques. However, increasing suspicion now falls on smaller, soluble A-beta complexes as the toxic form of the protein, partly through their ability to induce excess calcium influx into cells, which disrupts synaptic signaling and stimulates cell death. A new study in The Journal of Cell Biology (www.jcb.org) uses high-resolution imaging to reveal that A-beta oligomers elevate calcium by forming calcium-permeable pores in the plasma membrane.

A-beta oligomers could induce calcium influx by physically disrupting the cell's outer membrane or by activating endogenous calcium channels. But studies have also shown that A-beta peptides can form calcium-permeable pores themselves in both artificial and cell membranes. A limitation of experimental techniques used to date, says Angelo Demuro, from the University of California, Irvine, is that they only monitor the activity of one or two channels at a time. In addition, different groups have obtained disparate results regarding the properties of A-beta channels using this approach.

To overcome these problems, Demuro and colleagues developed an alternative method to measure the activity of calcium channels in living cells. "We can simultaneously record the behavior of thousands of channels using an imaging technique we call optical patch-clamping," Demuro explains. In this approach, frog eggs are filled with a calcium-sensitive dye, and the researchers observe the part of the cell nearest to the cell's outer membrane. When membrane channels open to let calcium into the cell, small fluorescent flashes indicate the duration and extent of calcium influx at each individual pore.

Demuro et al. found that, just twenty minutes after A-beta oligomers were added to the eggs, they displayed flickering spots of fluorescence signifying calcium influx through single membrane channels. This influx was unlikely to be through endogenous channels activated by A-beta because frog eggs barely express calcium channels of their own. Moreover, A-beta aggregates weren't simply disrupting the eggs' membrane, as the influx was inhibited by zinc ions, which block calcium-permeable pores.

A-beta oligomers therefore form calcium-permeable channels of their own in the membrane. Demuro and colleagues characterized the properties of these pores by simultaneously imaging the activity of thousands of channels in a single membrane region. "They are all different," says Demuro. "[The pores] show a wide variety of behaviors." Most pores opened infrequently and only let in small amounts of calcium, but some opened more often and channeled large amounts of calcium into the cell. Though few in number, Demuro et al.'s measurements suggest that this latter type of pore may be largely responsible for the toxic increase in cytoplasmic calcium levels.

Differences in the properties of individual pores may be caused by differences in the number of A-beta peptides assembled into each channel, with higher-order oligomers forming the more active species of pore. "It would be nice to visualize how many A-beta peptides each pore has and whether this is related to the activity of the channel," Demuro says. If pore activity is affected by the oligomerization state of A-beta, it appears that A-beta peptides continue to assemble after their insertion into membranes, as the pores became more active as eggs were exposed to A-beta oligomers for longer periods. This increase in calcium influx over time may be related to the gradual progression of Alzheimer's symptoms.

Beyond Alzheimer's disease, Demuro et al.'s approach may help explain the pathogenesis of other neurodegenerative disorders like Parkinson's and Huntington's disease, in which misfolded and aggregated proteins have also been reported to form calcium-permeable channels.
-end-
About The Journal of Cell Biology

Founded in 1955, The Journal of Cell Biology (JCB) is published by The Rockefeller University Press. All editorial decisions on manuscripts submitted are made by active scientists in conjunction with our in-house scientific editors. JCB content is posted to PubMed Central, where it is available to the public for free six months after publication. Authors retain copyright of their published works and third parties may reuse the content for non-commercial purposes under a creative commons license. For more information, please visit www.jcb.org.
Demuro, A., et al. 2011. J. Cell Biol. doi:10.1083/jcb.201104133

Rockefeller University Press

Related Protein Articles from Brightsurf:

The protein dress of a neuron
New method marks proteins and reveals the receptors in which neurons are dressed

Memory protein
When UC Santa Barbara materials scientist Omar Saleh and graduate student Ian Morgan sought to understand the mechanical behaviors of disordered proteins in the lab, they expected that after being stretched, one particular model protein would snap back instantaneously, like a rubber band.

Diets high in protein, particularly plant protein, linked to lower risk of death
Diets high in protein, particularly plant protein, are associated with a lower risk of death from any cause, finds an analysis of the latest evidence published by The BMJ today.

A new understanding of protein movement
A team of UD engineers has uncovered the role of surface diffusion in protein transport, which could aid biopharmaceutical processing.

A new biotinylation enzyme for analyzing protein-protein interactions
Proteins play roles by interacting with various other proteins. Therefore, interaction analysis is an indispensable technique for studying the function of proteins.

Substituting the next-best protein
Children born with Duchenne muscular dystrophy have a mutation in the X-chromosome gene that would normally code for dystrophin, a protein that provides structural integrity to skeletal muscles.

A direct protein-to-protein binding couples cell survival to cell proliferation
The regulators of apoptosis watch over cell replication and the decision to enter the cell cycle.

A protein that controls inflammation
A study by the research team of Prof. Geert van Loo (VIB-UGent Center for Inflammation Research) has unraveled a critical molecular mechanism behind autoimmune and inflammatory diseases such as rheumatoid arthritis, Crohn's disease, and psoriasis.

Resurrecting ancient protein partners reveals origin of protein regulation
After reconstructing the ancient forms of two cellular proteins, scientists discovered the earliest known instance of a complex form of protein regulation.

Sensing protein wellbeing
The folding state of the proteins in live cells often reflect the cell's general health.

Read More: Protein News and Protein Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.