Imaging techniques promise to revolutionize the diagnosis of Alzheimer’s and other neurodegenerative diseases, but at this point, data on how live images correlate with pathological brain changes are still limited. In the December 13 Brain, researchers led by Agneta Nordberg of the Karolinska Institute in Stockholm, Sweden, report on the postmortem examination of the first-ever AD patient to be followed by positron emission tomography of Pittsburgh Compound B (PIB-PET). In common with the handful of prior such analyses, Nordberg and colleagues found that PIB retention accurately identified fibrillar amyloid deposits. In addition, the authors performed detailed studies not done before and turned up intriguing correlations between amyloid accumulation and synaptic receptor density, as well as a surprising lack of correlation with markers of inflammation. The data from this patient help point the way toward fruitful areas of investigation for future postmortem studies.
“Although there are several correlative studies in the literature, few are as detailed and comprehensive as this one,” wrote Bill Klunk of the University of Pittsburgh, Pennsylvania, in an e-mail to ARF (see full comment below). “It will continue to be of value to perform exquisitely detailed postmortem in vivo correlations such as this in many more brains.”
The first such study, in 2007, verified that in vivo PIB imaging matched actual fibrillar Aβ deposits in a patient with dementia with Lewy bodies (see ARF related news story on Bacskai et al., 2007). In 2008, these findings were confirmed in an AD patient (see ARF related news story on Ikonomovic et al., 2008).
The current paper analyzed the brain of a woman with AD who had two copies of the ApoE4 allele. She died at the age of 61. In February 2002, at the age of 56, she volunteered for the first PET-PIB scan ever performed (see ARF related news story on Klunk et al., 2004). She received another PIB scan two years later, and over the course of her disease also got an MRI and three PET scans using fluorodeoxyglucose (FDG), a marker for glucose use and therefore brain metabolism.
These longitudinal data are one of the things that sets this case study apart, Klunk noted, as they allow researchers to look at disease progression. Over the eight years she was studied, the woman’s score on the Mini-Mental State Examination declined from a near-normal score of 27 down to five. The FDG data showed that her brain’s glucose metabolism decreased in parallel with her cognitive powers. By contrast, PIB retention, already high at first examination, showed little change over two years during which her cognition declined steeply. The amount of amyloid deposition seen at autopsy three years later also looked similar to PIB estimates, Nordberg said, suggesting no further change in amyloid between the second PIB scan and death three years later. This pattern matches the data from numerous previous studies, in which PIB retention increases during mild cognitive impairment, then seems to plateau during AD (e.g., see five-year study by Kadir et al., 2010).
Autopsy results confirmed the patient’s diagnosis of pure AD. First authors Ahmadul Kadir and Amelia Marutle validated the accuracy of PIB imaging by staining slices from several brain regions with five different anti-Aβ antibodies and a tau antibody. They also examined PIB binding to brain homogenates from nine regions, and compared both autopsy methods to PIB imaging data. The results confirmed that in vivo PIB retention correlates quite well with amyloid deposits, but does not correlate closely with tau and neurofibrillary tangles, as previous studies have found (see Ikonomovic et al., 2008).
To look at synaptic changes, Kadir and colleagues performed binding assays on brain homogenates with markers specific for two different subunits of the nicotinic acetylcholine receptor. Previous studies found that nicotinic receptor density is lower in AD brains than in healthy people (see Paterson and Nordberg, 2000), and that the decline in nicotinic receptors correlates with cognitive decline (see Kadir et al., 2006). Intriguingly, Kadir and colleagues found that the density of the α4β2 nicotinic receptor subunit was lower in brain regions with high amyloid, suggesting an interaction. They found no correlation between amyloid and the density of the α7 subunit, however. The results “indicate that [specific] nicotinic receptors might be closely involved with amyloid processes,” Nordberg said. “That might open up new strategies for developing drugs.”
Gerhard Koenig at drug discovery company EnVivo Pharmaceuticals in Watertown, Massachusetts, pointed out that if α4β2 subunits disappear over the course of the disease, then drugs targeting those subunits are less likely to work late in the disease course. Four clinical trials with compounds targeting α4β2 for AD have turned up negative, Koenig added, which is consistent with the new findings. However, the results from this patient need to be repeated in many more brains, Koenig said, and in people with other ApoE genotypes, in order to get the full picture. Some α7 agonists are currently in clinical trials (e.g., this EnVivo Phase 2 trial).
The data on inflammation are more puzzling. Using an antibody to glial fibrillary acidic protein, which labels astrocytes, Kadir and colleagues found increased numbers of astrocytes populating areas with lots of amyloid. This fits with previous findings of increased astrocytosis in AD brains. However, when the authors used PET ligands specific for activated astrocytes and activated microglia, they saw no correlation between these cell types and areas of high amyloid deposition. This conflicts with some in vivo data (e.g., Edison et al., 2008). One possibility, the authors suggest, is that autopsy tissue simply does not bind these ligands well. Another explanation, Nordberg said, is that astrocytes are present but not activated. She also speculated that inflammation might be more pronounced earlier in AD, and therefore does not correlate well with amyloid at autopsy. Koenig suggests, on the other hand, that by late stages of the disease, the baseline inflammation of the brain might be so high that no further elevation can be seen in areas of high amyloid.
One limitation of PET-PIB is that it only measures fibrillar Aβ. Nordberg compared fibrillar, extracellular amyloid to snow that piles up outside a house: It can be quite deep and still cause no problems for those inside. Soluble, oligomeric Aβ, on the other hand, might be more like water that gets inside a house and causes extensive damage. “[Oligomers] would probably correlate much more with functional activity and cognitive impairment,” Nordberg said. She is working on methods to try to image oligomers in living brains. Nordberg also points to the value of studying things such as inflammation and synaptic function in living patients, as she believes this data will be crucial for the evolution of new drug therapies.—Madolyn Bowman Rogers
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