Amyloid-beta Clumping Causes Cells to ‘Fry Like an Egg’
The toxic clumping of amyloid-beta, one of two key proteins implicated in Alzheimer’s disease, causes living human cells to generate heat, which may cause more clumps to form, a study suggests.
“Overheating a cell is like frying an egg — as it heats up, the proteins start to clump together and become non-functional,” Gabriele Kaminski Schierle, PhD, the study’s lead researcher and a professor at the U.K.’s University of Cambridge, said in a press release.
The researchers used sensitive sensors that can detect temperature changes inside cells. This tool may help diagnose Alzheimer’s or be used as a method to screen therapeutic candidates, the researchers noted, after testing a small molecule that was able to block amyloid-beta clumping and lower cell temperature.
The study, “Intracellular Aβ42 Aggregation Leads to Cellular Thermogenesis,” was published in the Journal of the American Chemical Society.
Many neurodegenerative diseases are characterized by protein misfolding and clumping, or aggregation, with aging as a leading risk factor.
In Alzheimer’s disease, the proteins amyloid-beta and tau form aggregates, called plaques and tangles, which cause brain cells to die, resulting in memory loss and other associated symptoms.
However, it is still unclear what biochemical changes occur inside a cell that lead to amyloid-beta plaques, also referred to as fibrils.
Recent studies suggest energy-intensive areas in cells, known as hotspots, occur as the cell ages or during cellular stress. Intracellular thermogenesis is the process and analysis of heat generation within cells, but experiments to understand this process have not been applied to Alzheimer’s disease.
Because the formation of amyloid-beta fibrils is known to release energy in the form of heat (exothermic), it has been suggested that this heat promotes further protein aggregation that eventually spreads to adjacent healthy cells, causing disease.
“As the fibrils start elongating, they release energy in the form of heat,” Kaminski Schierle said. “Amyloid-beta aggregation requires quite a lot of energy to get going, but once the aggregation process starts, it speeds up and releases more heat, allowing more aggregates to form.”
Chyi Wei Chung, first author of the study and a PhD candidate under the supervision of Kaminski Schierle, added: “Once the aggregates have formed, they can exit the cell and be taken up by neighbouring cells, infecting healthy amyloid-beta in those cells.”
Kaminski Schierle and her colleagues used tiny temperature sensors called fluorescent polymeric thermometers to explore the link between temperature and protein aggregation within living cells.
In the first set of experiments, the team exposed a human cell line to amyloid-beta to trigger the aggregation process. As controls, cells were also treated with FCCP, a chemical known to induce heat, as well as MJ040X, a potential Alzheimer’s therapeutic designed to block aggregation by binding to amyloid-beta.
The team found that adding amyloid-beta increased the temperature within cells by 2.8 C (or 5 F). At the same time, imaging analysis confirmed the formation of fibrillar structures dispersed throughout the cell.
In contrast, with the addition of MJ040X, which reduces the amyloid-beta aggregation, temperatures reverted to that of the untreated control. Further tests showed that MJ040X suppresses heat generation caused by the growth of fibrils rather than the initial aggregation event.
“No one has shown this link between temperature and aggregation in live cells before,” Chung said.
Because heat may also come from impaired mitochondria — which provide energy to cells — the team designed experiments to uncouple mitochondria processes from protein aggregation.
The results showed that amyloid-beta aggregation caused a significant loss in ATP, the energy-carrying molecule produced by mitochondria, indicating functional impairment. The addition of MJ040X mitigated this loss. ATP also was lost with the addition of FCCP, but not with MJ040X alone.
“Combining this with intracellular thermometry measurements, we conclude the main heating effect seen in cells with added [amyloid-beta] derives from the exothermic effects of amyloid elongation rather than from mitochondrial damage, the latter of which is present but does not affect thermogenesis in our case,” the researchers wrote.
“Thermogenesis has been associated with cellular stress, which may promote further aggregation,” Chung said. “We believe that when there’s an imbalance in cells, like when the amyloid-beta concentration is slightly too high and it starts to accumulate, cellular temperatures increase.”
Computational modeling to complement the lab experiments suggested that heat dissipation is suppressed by the expansion of the amyloid-beta fibrillar structure.
“We further validate that intracellular thermometry can be used as a tool to study [amyloid-beta] aggregation and efficacy of an anti-aggregation small compound drug in a live cell model, thereby highlighting the potential of MJ040 as a therapeutic drug in [Alzheimer’s disease],” the researchers wrote.