Scientists have yet to find the cure for Alzheimer's disease, but there is a growing body of research focused on early diagnosis and tracking the disease's progression.
A new University of California Berkeley study suggests for the first time that positron emission tomography (PET) scans can be used to follow the progressive stages of the neurodegenerative disease, even in adults who exhibit no symptoms.
Led by Dr. William Jagust, the research team performed PET scans on 53 adults: five young adults who were 20 to 26 years old; 33 retired and healthy adults who were 64 to 90 years old; and 15 retired adults aged 53 to 77 who already showed signs of Alzheimer's.
In the process, the UC Berkeley team looked for the presence of two key proteins - beta-amyloid and tau - which have been linked to the disease in previous studies.
High Levels Of Tau In The Brain
Scientists had considered the buildup of beta-amyloid plaques as the main culprit in the development of Alzheimer's disease, but in recent years, studies found that the microtubule protein called tau has a major role in it. When tau proteins get tangled, its ability to support synaptic connections is damaged.
Prior to the new study, the accumulation of these two proteins has been examined by German researchers in the brains of people who have already passed away. The stages of buildup were assigned based on a scale from one to six.
However, no one had been able to show the buildup stages - which is named as the Braak Staging after Heiko and Eva Braak - among people who are still alive. Thanks to PET scans, it is now possible.
Through PET scans, Jagust and his colleagues found that tau protein actually forms in the brain's medial temporal lobe. This region is responsible for storing memories and is home to the hippocampus.
"Tau is basically present in almost every aging brain," said Jagust's co-author Michael Schöll, a neuroscientist from Sweden.
Schöll said almost all of the elderly in the study had tau in their brain. In their case, the accumulation of tau was independent of amyloid, and could have been driven by aging, he said.
The team tested the participants' episodic memory by asking them to recall a list of words they viewed 20 minutes before. Episodic memory is the type of memory that regulates new information. Researchers found that higher levels of tau in the brain, especially in the medial temporal lobe, were linked to decline in episodic memory.
The Relationship Between Tau And Beta-Amyloid
The research team wonders why some people with high levels of tau in their brain do not often develop Alzheimer's later in life. Additionally, adults who may have beta-amyloid in their brains still end up being cognitively healthy.
Samuel Lockhart, another member of the research team and a postdoctoral fellow at UC Berkeley, said it does not mean that one protein is more vital than the other.
"Our study suggests that they may work together in the progression of Alzheimer's," said Lockhart.
It may be that amyloid facilitates the spread of tau in the medial temporal lobe to other regions in the brain such as the neocortex. However, Jagust said they have yet to find out how amyloid affects tau proteins and vice versa.
"All I can say is that when amyloid starts to show up, we start to see tau in other parts of the brain, and that is when real problems begin," said Jagust. "We think that may be the beginning of symptomatic Alzheimer's disease."
Meanwhile, Jagust believes that PET scans can be used as a tool for early diagnosis and staging. It can open up the possibility of developing therapeutic treatments that target the protein, largely depending on the stage of the disease.
The team's findings are featured in the journal Neuron. It was funded by the National Institutes of Health.