Deciphering the genesis and fate of amyloid β-protein yields novel therapies for Alzheimer disease (original) (raw)

If Aβ peptides, particularly Aβ42, are overproduced or insufficiently cleared, they become prone to aggregation into stable oligomers and larger polymers, apparently culminating in mature amyloid fibrils. In vitro aggregation studies of pure, synthetic Aβ peptides of a single length indicate that the critical concentration that needs to be reached for aggregation to begin is relatively high, perhaps in the mid-micromolar range (52). In certain cell culture models, far lower (low nanomolar) levels of the heterogeneous Aβ species, generated naturally by cells are associated with the appearance of small amounts of stable oligomers (53). This observation suggests that pro- (or anti-) aggregating cellular factors may regulate the conversion of monomers to dimers and higher oligomers. In vivo, it is assumed that high local concentrations of Aβ sufficient to initiate and propagate oligomers are achieved in certain brain areas, eventually leading to the formation of microscopically visible deposits (plaques). Such high local levels have been demonstrated in the cortex of both AD patients and APP transgenic mice (54, 55). Many of the resultant in vivo deposits appear amorphous or diffuse, indicating that the Aβ occurs principally in nonfibrillar (fine granular) aggregates. Whether some of this “pre-amyloid” material is similar to the protofibrils that form as metastable intermediates during synthetic Aβ fibrillization in vitro (56, 57) remains to be determined.

A central question about the Aβ hypothesis concerns which assembly forms of the peptide — monomers, oligomers (e.g., protofibrils), and/or mature amyloid fibrils — may be neurotoxic. Although synthetic Aβ fibrils reproducibly induce neuronal injury and loss in cell culture and after intracortical injection (58), the aggregates employed are usually complex mixtures of assembly forms that are difficult to define and quantify individually. The occurrence of neuritic dystrophy, microglial activation, and astrocytosis within amyloid plaques has long supported the hypothesis that these mature, fibrillar lesions can induce injury. However, the fibrils are likely to be in equilibrium with abundant oligomers and monomers in their immediate vicinity, since amyloid fibrils have both an on-rate and an off-rate. In postmortem AD brain tissue and the brains of older APP transgenic mice, the complex mixture of Aβ assembly forms precludes assignment of toxic effects to a particular species. However, young APP transgenic mice show altered synaptic morphology and electrophysiological changes well before the microscopic appearance of Aβ deposits (59, 60). Because such mice show steadily rising total Aβ levels in the brain before plaques develop (55), Aβ monomers and/or oligomers could be responsible.

In an attempt to discern which species of Aβ may be synaptotoxic in vivo, we have recently taken advantage of the production by certain cultured cells of low levels (<1 nM) of highly stable oligomers (dimers, trimers, and tetramers) of naturally secreted human Aβ (53). Microinjection of the conditioned media into the lateral ventricle of anesthetized rats potently blocked the maintenance of hippocampal long-term potentiation (LTP), an electrophysiological correlate of synaptic plasticity (61). This medium is devoid of Aβ fibrils or protofibrils. Immunodepletion of all Aβ species fully reversed the block of LTP. To separate the effects of monomers and oligomers, we used the ability of IDE to quantitatively degrade Aβ monomers while leaving the oligomers unaltered (45). When IDE-treated medium devoid of monomers was microinjected, LTP was still blocked, enabling us to attribute the inhibition to the oligomers. This was confirmed by treating the same cells with doses of a γ-secretase inhibitor sufficient to lower oligomer levels to undetectable levels while preserving a substantial fraction (∼60%) of the monomer. Microinjection of this medium allowed normal LTP. Therefore, natural oligomers of human Aβ, in the absence of monomers and amyloid fibrils, can disrupt synaptic plasticity in vivo at concentrations found in human brain and CSF. It remains to be seen whether such a phenomenon occurs in the AD hippocampus and could result in synaptic dysfunction — and thus memory impairment — prior to widespread neuronal death.

While these and other data suggest a direct action of Aβ oligomers on synapses, Aβ aggregates could also lead indirectly to neurotoxicity by activating microglia and astrocytes and inducing a local inflammatory response, including the triggering of the complement cascade (reviewed in ref. 62). It is not yet clear whether microglia are the earliest cellular responders to the accumulation of Aβ or whether neurons react just as quickly. If microglia represent the first cellular target, some of the subsequent synaptic and dendritic changes in AD cortex may be caused by inflammatory and neurotoxic factors released by microglia. During the long presymptomatic phase of AD, it is likely that many molecular and cellular changes develop contemporaneously. As a result, attempts to identify one or a few “master” pathways of AD-type neurotoxicity and inhibit them pharmacologically may well prove elusive. Given the difficulty of ascertaining the precise sequence of biochemical changes, even in mouse models of the disease, neuronal and glial toxicity, per se, are less attractive therapeutic targets than is lowering the levels of Aβ monomers and oligomers.