A brake on overactive microglia
If CD33 is the yin, a gene called TREM2 is the yang. Discovered a few years after CD33, TREM2 reins in microglial activation, returning them to their role as cellular housekeepers.
Neurologist David Holtzman, MD, of Washington University in St. Louis, who studies TREM2, agrees that wherever you find amyloid, tau, or dead brain cells, there are microglia raring to go and ready to scavenge.
“I think at first a lot of people thought these cells were reacting to Alzheimer’s pathology, and not necessarily a cause of the disease,” he said.
It was the discovery of TREM2 on the heels of CD33 that really shifted the thinking, in part because it produces a protein that in the brain is only found in microglia. “Many of us [in the field] immediately said, ‘Look, there’s now a risk factor that is only expressed in microglia. It must be that innate immune cells are important in some way in the pathogenesis of the disease,’ “ he added.
Dr. Holtzman sees microglial activation in impending dementia as a double-edged sword. In the beginning, microglia clear unwanted amyloid to maintain brain health. But once accumulated amyloid and tau have done enough damage, the neuroinflammation that comes with microglial activation does more harm than good. Neurons die en masse and dementia sets in.
But not all researchers are convinced.
Serge Revist, PhD, is a professor in the department of molecular medicine at the Laval University Medical School in Quebec. Based on his lab’s research, he believes that while impaired immune activity is involved in Alzheimer’s disease, it is not the root cause. “I don’t think it is the immune cells that do the damage, I still think it is the beta-amyloid itself,” he said, “In my lab, in mouse studies, we’ve never found that immune cells were directly responsible for killing neurons.”
He does believe that in some patients with Alzheimer’s disease, microglia may not be able to handle the excess amyloid that accumulates in the disease and that developing treatments that improve the ability of microglia and the immune system to clear the protein could be effective.
Microglial medicines
The biological cascade leading to Alzheimer’s disease is a tangled one. Gene variants influencing the accumulation and clearance of amyloid are likely a major contributor. But immune activity caused by early life infection might also be involved, at least in some cases. This infectious theory of Alzheimer’s disease was first proposed by Dr. Tanzi’s now-deceased colleague Robert Moir, PhD. Dr. Tanzi’s group even has evidence that amyloid itself is antimicrobial and evolved to protect us from pathogens, only to become a problem when overactive and aggregated.
And the same goes for microglia, cells whose over-ambition might cause much of the brain degeneration seen in Alzheimer’s disease.
In theory, if a treatment could decrease CD33 activity or increase that of TREM2, doctors might one day may be able to slow or even stop the progression of dementia. Instead of going after amyloid itself – the mechanism behind so many failed investigational Alzheimer’s drugs – a therapy that quells the immune response to amyloid might be the answer in treating dementia.
“There are a number of scientists and companies trying to figure out how to influence genes like TREM2 and CD33 and to both decrease amyloid and act on the downstream consequences of the protein,” said Dr. Holtzman. “All of this is to say that somewhere in the biology that causes Alzheimer’s disease, the immune system is involved.”
It seems that in many cases, the most common form of a dementia might be due to a well-intentioned immune cell going rogue. “I think you’d hear this from basically any researcher worth their salt,” said Dr. Tanzi. “I feel strongly that without microglial activation, you will not get Alzheimer’s disease.”
A version of this article first appeared on Medscape.com.