The availability of specific antibodies that can recognize prefibrillar oligomers (A11), fibrils and fibrillar oligomers (OC) and annular protofibrils (aAPF) has allowed a detailed analysis of the levels and distributions of these distinct types of soluble oligomers in AD in individuals with plaque and tangle pathology but without dementia and nondemented brains with low or undetectable levels of pathology. We found significantly elevated levels of soluble fibrillar oligomers in AD patients compared to SDC and controls. Elevated soluble fibrillar oligomers were present in all frontal cortex regions tested, as well as the hippocampus, entorhinal cortex, transentorhinal cortex and cerebellum. With the exception of the cerebellum, each of these regions is known to develop neuropathology over the course of AD progression. We determined that the elevated levels of fibrillar oligomers were not an artifact created by homogenization of plaque-containing tissue because regions of control and SDC tissue that have high levels of insoluble fibrillar plaques did not have correspondingly high level of soluble fibrillar oligomers. Further, the soluble fibrillar oligomers were found not to be a product of increased expression of APP or Aβ. Having ruled these possibilities out, we can be confident that the fibrillar oligomers reported here are indeed a distinct structure and aggregation state present primarily in AD brain.
The only brain region examined that did not exhibit elevated levels of soluble fibrillar oligomers in AD was the olfactory bulb. The olfactory bulb fibrillar oligomer levels are elevated in SDC and age matched normal control brain, compared to other regions of the same brain. Neurons in the olfactory bulb are known to degenerate in AD and this is associated with the focal accumulation of Aβ (Struble and Clark, 1992
). The elevation of fibrillar oligomers in the olfactory bulb is potentially interesting for two reasons. The first is that olfactory function declines with age overall (Schiffman, 1997
), and overall high levels of soluble fibrillar oligomers may represent an aging phenomenon. Thus, one extension of the current study would be to examine younger individuals with the prediction that levels of oligomers would be low or undetectable. Second, there is evidence that a loss of smell may be an early indicator of incipient AD and is detectable in individuals that go on to develop mild cognitive impairments (Gilbert and Murphy, 2004
; Wilson et al., 2007
). The potential correlation between olfactory bulb fibrillar oligomer levels and olfactory deficits warrants further investigation.
The finding that soluble fibrillar oligomer levels are elevated in cerebellum is interesting because this brain region is not known to be functionally impaired in AD. Amyloid plaques have been reported in the cerebellum of AD patients, although this region does not exhibit neurofibrillary tangle pathology (Braak et al., 1989
). Lacor et al. have also reported elevated levels of Aβ oligomers in AD brain, but the levels were not statistically significantly elevated compared to controls (Lacor et al., 2004
). The reasons for the discrepancy between the significantly elevated levels we observe are not entirely clear, but could be due to differences in the selectivity of the antibodies used. If soluble fibrillar oligomers are causally related to neuronal dysfunction, the fact that their levels are elevated in the cerebellum suggests that cerebellar neurons may be less susceptible to their toxic activity. This is consistent with previous reports of the resistance of cerebellar neurons to aggregated Aβ in vitro (Ueda et al., 1994
We also found an inverse correlation between soluble fibrillar oligomers and MMSE score. With increasing levels of fibrillar oligomers in brain regions associated with intact memory function, decreased cognitive function was evident. We also found significant correlations between fibrillar oligomer levels and the neuropathological hallmarks of AD, amyloid plaques and tau tangles. Considering the variable reports of both a lack of correlation between insoluble fibrillar aggregates or a high correlation with cognitive dysfunction, the correlation of soluble fibrillar oligomers we observe in the current study to the clinical and neuropathological progression of AD points to this species as being potentially a key factor involved with neurodegeneration and thus a potential therapeutic target.
In contrast, we found that prefibrillar oligomer and annular protofibril levels do not correlate with cognitive dysfunction in AD brain. We previously reported that A11 positive prefibrillar oligomers are detected in human AD brain (Kayed et al., 2003
), but the quantitative analysis conducted here indicate that they are also found in age-matched non demented control brain. Prefibrillar oligomers are toxic in vitro (Demuro et al., 2005
; Kayed et al., 2003
), so if they intrinsically toxic, this suggests that something that is present in brains of non-demented individuals may modulate their toxicity. Since prefibrillar oligomers are kinetic intermediates in fibril formation (Kayed et al., 2003
), it is possible that their conformational conversion to fibrillar oligomers is important for toxicity and AD pathogenesis. Prefibrillar oligomers appear to be precursors for annular protofibril levels. Since prefibrillar oligomer levels are elevated in age matched control brain, it is perhaps not surprising that annular protofibril levels are also not correlated with AD.
In summary, soluble fibrillar oligomers are specifically correlated with pathology and cognitive dysfunction in AD brain. Soluble fibrillar oligomers may represent a target for the development of immunological reagents or small molecules that specifically inhibit their formation or toxicity.