Early in Alzheimer’s, connections between neurons begin to wither and die. One of the defining features of AD, this synapse loss underlies cognitive impairment, but has proven hard to track in the living brain. A paper in the July 20 Science Translational Medicine may change that. Scientists led by Richard Carson and Sjoerd Finnema at Yale University in New Haven, Connecticut, report on a positron emission tomography (PET) tracer that binds to a protein on presynaptic vesicles, revealing the density of synapses throughout the brain. In healthy people, the compound entered the brain quickly and bound predominantly gray matter. In epilepsy patients, it picked up areas of synaptic loss near the focal point of seizures.
“This paper shows what appears to be the first radioligand to bind to sites at synapses in the brain,” wrote Richard Mohs, Global Alzheimer’s Platform Foundation, to Alzforum. If confirmed, this could provide a way to measure synaptic density in living people and track changes in density regionally and with disease, he added. “These scans would complement other newly developed radioligands that allow imaging of amyloid plaques, neurofibrillary tangles, and other types of brain pathology.”
Synaptic signal. [11C]UCB-J binds synapses and lights up in a PET scan of a healthy subject. [Science Translational Medicine/AAAS.]
Until now, researchers could only measure synaptic density in postmortem tissue by using antibodies to detect proteins such as synaptophysin (Buckley and Kelly, 1985). A molecule that binds synaptophysin might prove useful as a PET ligand but scientists have yet to find one. However, levetiracetam—an anti-epileptic drug—targets a similarly widespread protein called synaptic vesicle glycoprotein 2A (SV2A). Previous studies have reported that every synaptic vesicle in the brain carries two to five copies of this protein, said Finnema. UCB, the Brussels-based company that developed levetiracetam and provided some of the funding for this study, derived several PET tracer candidates based on this drug. A couple have been tested in animals and humans, but no reports on human brain imaging had been published (Estrada et al., 2016; Bretin et al., 2015). Carson, who directs the Yale PET Center, which specializes in developing and using new PET ligands, chose one of the derivatives, [11C]UCB-J, to test in people.
Using non-human primates first, the researchers found that PET scans lit up everywhere there was a functioning synapse (Nabulsi et al., 2016). To confirm that the [11C]UCB-J PET signal measured synapse density, Finnema and colleagues compared an in vivo PET scan of a baboon with postmortem analysis of its brain tissue The PET signal in 11 areas of gray matter closely matched the density of SV2A protein as measured in western blots. There was very little signal in the centrum semiovale, a white matter area, in either the PET scan or the westerns. Western blot analyses also revealed the densities of SV2A and synaptophysin tracked closely. Using high-powered confocal microscopy on a section of the somatosensory cortex, the authors found that both proteins turned up in dendrites, but not neuronal cell bodies. Combined, the data suggested that SV2A made an effective surrogate for synaptophysin. [11C]UCB-J bound SV2A with high affinity and at low nanomolar concentrations.
The Human Test
Would the tracer prove as effective in people? To find out, the researchers injected [11C]UCB-J intravenously into five healthy subjects, average age 37, and conducted PET scans using a High Resolution Research Tomograph, which resolves signals down to about 3mm, about twice as good as the typical PET scanner. The compound was effectively taken up by the brain, peaking in gray matter regions about 15 minutes after injection, meaning the entire process from injection to scan could conceivably take as little as an hour, Finnema said. Gray matter bound most of the tracer, contrasting starkly with the low uptake in white matter regions such as the centrum semiovale. Using the latter as a reference region gave peak standard uptake value ratios (SUVRs) of seven to 11 in cortical regions (see image above). Studies in three more healthy subjects suggested UCB-J was indeed binding SV2A; when the researchers injected levetiracetam halfway through the imaging procedure, the signal diminished, indicating that the two compounds compete for the same binding site.
To test if the ligand could be used to detect synaptic loss, the researchers then turned to three patients, average age 52, with epilepsy. All had mesial temporal lobe sclerosis on one side of the brain, meaning neurons had died away where the seizures originated, near the hippocampus. Compared to the healthy side, the affected hippocampus of each patient bound 40 to 60 percent less [11C]UCB-J (see image below). That signal loss matched the atrophy visible in MRI.
According to Carson, the one big downside to the tracer is that it is labeled with C11, which has a half-life of only 20 minutes. This means [11C]UCB-J will be limited to centers with a nearby cyclotron for now. However, UCB-J contains three fluorine atoms, meaning it can, in principle, be labeled by F18, which has a half-life of almost two hours, said Carson. The C11 version is approved by the FDA for research purposes, but researchers will wait for F18-labeled compounds to seek approval for broader clinical use.
The authors are now working out which area of the brain would best serve as a reference region for generating SUVRs. It could be the centrum semiovale, which according to western blots should have no SV2A binding. However, the displacement PET studies with levetiracetam hinted at small amount of binding there.
“The authors have checked off the list of essential properties required of a useful PET radiopharmaceutical, and [11C]UCB-J has passed nearly all with flying colors,” wrote Chet Mathis, University of Pittsburgh, to Alzforum (see full comment below). The ligand could help determine how early in neurodegenerative diseases synaptic losses appear and how important assessment of synaptic losses will prove to be, he added. However, he cautioned that using the relatively small centrum semiovale as a reference region could be problematic in clinical PET scanners that have less than the relatively sharp 3mm resolution of the scanner used in this study.
“This looks like a promising tracer,” said Reisa Sperling, Brigham and Women’s Hospital, Boston. It would be important to see how it performs across the spectrum of early AD, to see at what stage synaptic density changes, she said. She also wondered what this tracer might detect before that loss, when synapses are hyperactive. Regardless, [11C]UCB-J might provide a specific way to track disease progress and tell whether drugs slow or reverse it. “So far we can’t regrow neurons, but maybe we can remodel and regrow synapses,” she told Alzforum. “I think the potential of being able to pick up a therapeutic effect before neurons are dead is important.”
Carson’s group is now scanning patients with AD and MCI, as well as older controls, to see how [11C]UCB-J binding compares to that seen in younger controls. Co-authors are also collaborating with other labs to image patients with schizophrenia, depression, Parkinson’s disease, traumatic brain injury, and other disorders with autopsy evidence of synapse damage. “If we can monitor synapse loss in living people, and see the time course and the effect of drugs, the possibility of impact on these diseases is dramatic,” Carson told Alzforum. He estimates that, given the interest he’s seen at conferences, the tracer will likely be used at other centers in the next 6 to 12 months. “We’re excited to add this into the armamentarium of imaging in AD. It could be an extremely important tool to help understand how amyloid and tau deposition interact with synaptic loss and lead to cognitive deficits.” Scientists led by Keith Johnson at MGH recently uncovered sites in the brain where tau and Aβ pathologies might interact to exacerbate the spread of disease (see Jul 2016 news).
Arthur Toga, University of Southern California, Los Angeles, pointed out that subjects can receive only so many radioactive tracers at a time, but argued in favor of as full a characterization of AD pathophysiology as possible. This tracer may enable scientists to better measure changes in synaptic density that relate to tissue atrophy changes seen using magnetic resonance imaging, he said.
Lennart Mucke, Gladstone Institute of Neurological Disease, San Francisco, California, agreed that the tracer could be influential. “Being able to non-invasively monitor the effects of neurodegenerative disorders and related therapeutics on synapse density could be a great advantage,” he wrote to Alzforum. He called the SV2A ligand promising, suggesting it could become a particularly meaningful and sensitive biomarker for Alzheimer’s disease. However, he agreed with the authors and other commentators that these results will have to be validated in larger numbers of people and in relevant patient populations.—Gwyneth Dickey Zakaib
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