Preliminary research suggests it may someday be possible to diagnose and forecast risk for Alzheimer’s disease using skin cells, thanks to a small protein, or peptide, that few previously associated with the disease.
Researchers at Washington University School of Medicine in St. Louis have discovered that skin cells from individuals with inherited forms of Alzheimer’s disease respond to the peptide bradykinin by triggering Alzheimer’s-like changes, whereas skin cells from healthy individuals do not. They found the same effect in skin cells from two young adults at high risk for developing the disease but who were too young to have clinical symptoms.
The study, published recently in The FASEB Journal and cited in the Internet-based Alzheimer’s Research Forum, also shows that bradykinin triggers a cascade of molecular events in these cells.
“No one has looked in-depth at the potential role of bradykinin in Alzheimer’s disease,” says principal investigator Nancy Baenziger, Ph.D., associate professor of anatomy and neurobiology. “Our findings need to be explored further in cases of non-inherited Alzheimer’s disease, but this preliminary evidence is very encouraging. It’s possible that these results could eventually lead to a way of determining an individual’s risk of developing Alzheimer’s before clinical symptoms arise, and that the bradykinin cascade could serve as a potential new drug target.”
Most efforts to develop diagnostic tests for Alzheimer’s disease focus on changes in the brain itself. But according to Baenziger, recent studies suggest the disease is systemic, affecting the entire body while most severely damaging the central nervous system.
The team used a National Institute on Aging database of skin cells from healthy individuals and those with genetic forms of Alzheimer’s disease. They focused on bradykinin because its production is triggered by even minor injuries and blood coagulation. Also, bradykinin receptors have been identified both in skin cells and, more recently, in brain areas where Alzheimer’s disease first appears. The peptide is known to play a key role in the body’s response to brain injury and tissue inflammation, two major risk factors for Alzheimer’s disease.
Baenziger’s team previously identified several different forms of bradykinin receptors that they termed H, I and L. They then developed probes that specifically detect the I and L forms. In their current study, the team used these probes to calculate the relative number of these receptors in skin cells from Alzheimer’s patients.
In one experiment, the cells were treated with a chemical that activates an enzyme called protein kinase C (PKC). The team found PKC activation triggers the production of I and L receptors in Alzheimer’s skin cells: the number of these receptors increased by up to 450 percent in cells from Alzheimer’s patients but there was almost no change in skin cells from healthy individuals, who ranged in age from 16 to 82.
The team then carried out more extensive studies of skin samples grouped according to the genetic defects that may have caused the disease. Skin cells from patients with the two most common forms of genetically linked Alzheimer’s — Presenilin-1 or Presenilin-2 — all had significantly increased numbers of I and L receptors compared to healthy individuals of the same age.
Nancy Baenziger
More strikingly, similar large increases in the number of I and L receptors were found in skin cells from a 19-year-old and a 20-year-old. Both had Down syndrome, and individuals with Down syndrome frequently develop Alzheimer’s disease, typically around age 40.
“Our results from young adults with Down syndrome suggest that changes in bradykinin receptors may be detectable decades before clinical symptoms of Alzheimer’s disease appear,” Baenziger explains. “This is very exciting because early detection may ultimately prove to be key for preventing and treating the disease.”
The team also tested whether adding bradykinin itself to the skin cultures would increase the number of I and L bradykinin receptors. Indeed, there was a 250 percent increase in cells from the two individuals with Down syndrome and from people with Presinilin-1 Alzheimer’s. Similar increases also were observed in cells from a 77-year-old Alzheimer’s patient who had no presenilin mutations. There was no such change in cells from healthy individuals.
Both effects — increases in I and L receptors after stimulating PKC activation and after direct treatment with bradykinin — were blocked when the cells first were treated with a chemical that inhibits PKC.
“This evidence suggests a pathway of events, in which bradykinin activates PKC, which in turn triggers an increase in the number of these I and L receptors,” Baenziger says.
The team also identified another step in the molecular cascade and found evidence that this step is carried out by I and L receptors created during the earliest moments of the cascade.
One of the main characteristics of Alzheimer’s disease is thought to be the formation of tangled networks of a protein called tau in neurons. Overactivity of a biochemical process known as phosphorylation is likely one of the first steps in the development of these Alzheimer’s-associated tangles. Baenziger’s team found that treatment with bradykinin increased the amount of tau phosphorylation by about 250 percent in skin cells from Alzheimer’s patients and from the Down syndrome patients, but not in cells from healthy individuals. Moreover, the type of phosphorylation appeared similar to that seen in the brain during early Alzheimer’s disease.
Just as in the other experiments, treatment with a PKC inhibitor blocked the dramatic increase in tau phosphorylation.
“We have now shown that I and L bradykinin receptors are biochemically distinctive and target the tau protein that goes awry in Alzheimer’s disease,” Baenziger says. “These clues are telling us something about the environment inside cells vulnerable to Alzheimer’s, which may eventually allow us to create molecular profiles that identify individuals vulnerable to developing this disease.”
Jong YJI, Ford SF, Seehra K, Malave VB, Baenziger NL. Alzheimer’s disease skin fibroblasts selectively express a bradykinin signaling pathway mediating tau protein Ser phosphorylation. The FASEB Journal, Dec. 17, 2003. Cited at www.alzforum.org.
This research was supported through the Alzheimer’s Association and the Washington University Alzheimer’s Disease Research Center P50AGO5681.
The full-time and volunteer faculty of Washington University School of Medicine are the physicians and surgeons of Barnes-Jewish and St. Louis Children’s hospitals. The School of Medicine is one of the leading medical research, teaching and patient-care institutions in the nation. Through its affiliations with Barnes-Jewish and St. Louis Children’s hospitals, the School of Medicine is linked to BJC HealthCare.