The pharmaceutical industry has spent billions of dollars on futile clinical trials directed at treating Alzheimer’s disease by ridding brains of a substance called amyloid plaque. But the new findings have identified another mechanism, involving an entirely different substance, that may lie at the root not only of Alzheimer’s but of many other neurodegenerative disorders — and, perhaps, even the more subtle decline that accompanies normal aging.
The study, published Aug. 14 in the Journal of Neuroscience, reveals that with advancing age, a protein called C1q, well-known as a key initiator of immune response, increasingly lodges at contact points connecting nerve cells in the brain to one another. Elevated C1q concentrations at these contact points, or synapses, may render them prone to catastrophic destruction by brain-dwelling immune cells, triggered when a catalytic event such as brain injury, systemic infection or a series of small strokes unleashes a second set of substances on the synapses.
“No other protein has ever been shown to increase nearly so profoundly with normal brain aging,” said Ben Barres, MD, PhD, professor and chair of neurobiology and senior author of the study. Examinations of mouse and human brain tissue showed as much as a 300-fold age-related buildup of C1q.
The finding was made possible by the diligence and ingenuity of the study’s lead author, Alexander Stephan, PhD, a postdoctoral scholar in Barres’ lab. Stephan screened about 1,000 antibodies before finding one that binds to C1q and nothing else. (Antibodies are proteins, generated by the immune system, that adhere to specific “biochemical shapes,” such as surface features of invading pathogens.)
Comparing brain tissue from mice of varying ages, as well as postmortem samples from a 2-month-old infant and an older person, the researchers showed that these C1q deposits weren’t randomly distributed along nerve cells but, rather, were heavily concentrated at synapses. Analyses of brain slices from mice across a range of ages showed that as the animals age, the deposits spread throughout the brain.
“The first regions of the brain to show a dramatic increase in C1q are places like the hippocampus and substantia nigra, the precise brain regions most vulnerable to neurodegenerative diseases like Alzheimer’s and Parkinson’s disease, respectively,” said Barres. Another region affected early on, the piriform cortex, is associated with the sense of smell, whose loss often heralds the onset of neurodegenerative disease.
Other scientists have observed moderate, age-associated increases (on the order of three- or four-fold) in brain levels of the messenger-RNA molecule responsible for transmitting the genetic instructions for manufacturing C1q to the protein-making machinery in cells. Testing for messenger-RNA levels — typically considered reasonable proxies for how much of a particular protein is being produced — is fast, easy and cheap compared with analyzing proteins.
But in this study, Barres and his colleagues used biochemical measures of the protein itself. “The 300-fold rise in C1q levels we saw in 2-year-old mice — equivalent to 70- or 80-year-old humans — knocked my socks off,” Barres said. “I was not expecting that at all.”
C1q is the first batter on a 20-member team of immune-response-triggering proteins, collectively called the complement system. C1q is capable of clinging to the surface of foreign bodies such as bacteria or to bits of our own dead or dying cells. This initiates a molecular chain reaction known as the complement cascade. One by one, the system’s other proteins glom on, coating the offending cell or piece of debris. This in turn draws the attention of omnivorous immune cells that gobble up the target.
The brain has its own set of immune cells, called microglia, which can secrete C1q. Still other brain cells, called astrocytes, secrete all of C1q’s complement-system “teammates.” The two cell types work analogously to the two tubes of an Epoxy kit, in which one tube contains the resin, the other a catalyst.
“Our findings may well explain the long-mysterious vulnerability specifically of the aging brain to neurodegenerative disease,” he said. “Kids don’t get Alzheimer’s or Parkinson’s. Profound activation of the complement cascade, associated with massive synapse loss, is the cardinal feature of Alzheimer’s disease and many other neurodegenerative disorders. People have thought this was because synapse loss triggers inflammation. But our findings here suggest that activation of the complement cascade is driving synapse loss, not the other way around.”
In 2011, Barres co-founded a company, Annexon, to develop drugs that inhibit the complement cascade to treat Alzheimer’s, glaucoma, Parkinson’s, stroke, multiple sclerosis and several other neurodegenerative diseases characterized by massive synapse loss. Annexon has licensed multiple associated patent applications from Stanford, which filed them.
ABSTRACT – The decline of cognitive function has emerged as one of the greatest health threats of old age. Age-related cognitive decline is caused by an impacted neuronal circuitry, yet the molecular mechanisms responsible are unknown. C1q, the initiating protein of the classical complement cascade and powerful effector of the peripheral immune response, mediates synapse elimination in the developing CNS. Here we show that C1q protein levels dramatically increase in the normal aging mouse and human brain, by as much as 300-fold. This increase was predominantly localized in close proximity to synapses and occurred earliest and most dramatically in certain regions of the brain, including some but not all regions known to be selectively vulnerable in neurodegenerative diseases, i.e., the hippocampus, substantia nigra, and piriform cortex. C1q-deficient mice exhibited enhanced synaptic plasticity in the adult and reorganization of the circuitry in the aging hippocampal dentate gyrus. Moreover, aged C1q-deficient mice exhibited significantly less cognitive and memory decline in certain hippocampus-dependent behavior tests compared with their wild-type littermates. Unlike in the developing CNS, the complement cascade effector C3 was only present at very low levels in the adult and aging brain. In addition, the aging-dependent effect of C1q on the hippocampal circuitry was independent of C3 and unaccompanied by detectable synapse loss, providing evidence for a novel, complement- and synapse elimination-independent role for C1q in CNS aging.
Brian Wang is a Futurist Thought Leader and a popular Science blogger with 1 million readers per month. His blog Nextbigfuture.com is ranked #1 Science News Blog. It covers many disruptive technology and trends including Space, Robotics, Artificial Intelligence, Medicine, Anti-aging Biotechnology, and Nanotechnology.
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