A Stanford University-led research team has identified plasma proteins linked to declines in neurogenesis and impaired learning and memory.
Their work indicates that age-related declines in neurogenesis and cognitive function are driven in part by changes in levels of blood-borne proteins, suggesting that manipulating these protein levels could be useful in mitigating age-related cognitive decline, Tony Wyss-Coray, Stanford associate professor of neurology and neurological sciences and leader of the project, told ProteoMonitor.
In the study, which was described in a paper published this week in Nature, the scientists used a surgical procedure to connect the circulatory systems of pairs of old and young mice, allowing blood from the two animals to mix. This comingling produced a three-fold increase in the number of new nerve cells generated in the old mice while leading to a decline in neurogenesis in the young mice.
To determine what factors in blood might be responsible for these changes, the researchers had Rules-Based Medicine – now Myriad RBM – run plasma samples from the mice to measure the relative levels of 66 cytokines, chemokines, and other secreted signaling proteins. This analysis identified six factors – the chemokines CCL2, CCL11, CCL12, and CCL19 along with haptoglobin and β2-microglobulin – as having elevated levels in the plasma of unpaired older mice and in young mice sharing blood with older mice.
The researchers then tested the effects of administering CCL11 to mice, finding that injecting the protein to induce an increase in CCL11 plasma levels led to a significant decrease in neurogenesis. This decrease, they found, could be reversed by neutralization of plasma CCL11.
They also identified an age-linked increase in CCL11 in the plasma and cerebrospinal fluid of humans between 20 and 90 years of age, suggesting that the phenomenon may be conserved across species.
"It's an important paper," Richard Ransohoff, director of the Neuroinflammation Research Center at the Cleveland Clinic and author of a commentary accompanying the article, told ProteoMonitor. "This is a nice proof of principle. I don't think it's going to be the case that CCL11 is the master regulator of neurogenesis. That would be very surprising. But [the research] suggests that you could use this approach to go further and see what other factors could be regulators of neurogenesis."
Currently, Wyss-Coray said, his team is working with a panel of assays for roughly 300 secreted signaling proteins that they've developed in house in hopes of identifying additional blood-based analytes involved in neurogenesis. The researchers are using antibodies for the target proteins spotted on glass slides. They biotinylate the samples and then detect hits via fluorescent streptavidin conjugates.
"We're hoping to find factors that are actively pro-neurogenic," he said. "In this study we've identified several factors that seem to inhibit neurogenesis, but we would like to find some sort of rejuvenating factors that can make the brain function like a younger brain."
The researchers are confining their focus for now to secreted signaling factors out of a desire to reduce the complexity of the plasma proteome, Wyss-Coray said.
"People [studying the plasma proteome] are looking at so many different proteins and molecular species – especially considering all the possible post-translational modifications and the cleavages of proteins – and many of these fragments probably have no biological relevance, but with traditional mass spec you're just looking at everything," he said. "So we reasoned that if we just focused on proteins [that] cells use to talk to each other – secreted signaling proteins such as cytokines, chemokines, growth factors, and secreted receptors – we would have a much more restricted list of proteins to study."
The hope, he added, is that the researchers can take proteins identified in the discovery stage and then use bioinformatics tools like Ingenuity Pathway Analysis to determine what signaling pathways might be involved and the mechanism by which they are acting. "I think we need to take some time and develop some biology behind these proteins," he said.
The co-founder of Alzheimer's biomarker firm Satoris, which was purchased by Rules-Based Medicine in May (PM 5/17/2011), Wyss-Coray said his previous experience working to translate protein biomarkers to the clinic has given him an appreciation for the need to develop a solid biological understanding of potential markers.
"In 2007 we published a study of 18 proteins as a signature for Alzheimer's and it created a lot of hype that we might have a blood test for Alzheimer's," he said. "But it turned out to be difficult to translate it to a clinical platform, and I think in part that was because we hadn't really biologically validated those proteins."
Indeed, following the publication of the study in Nature Medicine, Satoris expected to launch a test based on the 18 proteins by the end of 2008 (PM 8/7/2008). Those proteins have made it into research panels now owned by Myriad RBM, but not into an FDA-approved diagnostic.
Wyss-Coray has continued his Alzheimer's research in addition to the neurogenesis work. His lab has developed assays for 550 human secreted signaling proteins which they are using to study the disease as well as other dementias. In July, his group published a study in Molecular & Cellular Proteomics on using plasma proteins to model key pathological markers of Alzheimer's, including levels of cerebrospinal fluid β-amyloid and tau.
Ransohoff said he saw the neurogenesis research as complementary to a paper published last year in PLoS Biology that used a genetic approach to identify the ratio between CD4 and CD8 T cells in blood as being strongly correlated to levels of neurogenesis in mice.
This finding "was quite a surprise," he said. "Because at first blush it just doesn't make any sense. The reason that it could potentially make sense is that CD4 and CD8 cells could make different amounts and qualities of cytokines, and, secondly the neurogenic niches of the brain cluster very closely around blood vessels and particularly in blood vessel areas that don't have a particularly high blood-brain barrier effect."
The Stanford work, Ransohoff said, " took things a bit further [by showing] that the blood factor that is affecting neurogenesis is a secreted plasma protein, not a cell, and [by identifying] the plasma protein."
The study didn't, however, address the mechanisms by which these blood factors might be affecting neurogenesis, he noted.
"That's something that we still need to figure out," Wyss-Coray said. "The fact that we found this chemokine just suggests that these chemokines probably have an important function in regulating neurogenesis. Is this a direct or indirect effect on [neural] stem cells? We don't know yet."
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