The progressive neurodegeneration of Alzheimer9s disease has been hypothesized to be mediated, at least in part, by beta-amyloid protein. A relationship between the aggregation state of beta-amyloid protein and its ability to promote degeneration in vitro has been previously suggested. To evaluate this hypothesis and to define a structure- activity relationship for beta-amyloid, aggregation properties of an overlapping series of synthetic beta-amyloid peptides (beta APs) were investigated and compared with beta AP neurotoxic properties in vitro. Using light microscopy, electrophoresis, and ultracentrifugation assays, we found that few beta APs assembled into aggregates immediately after solubilization, but that over time peptides containing the highly hydrophobic beta 29–35 region formed stable aggregations. In short-term neuronal cultures, toxicity was associated specifically with those beta APs that also exhibited significant aggregation. Further, upon the partial reversal of beta 1–42 aggregation, a concomitant loss of toxicity was observed. A synthetic peptide derived from a different amyloidogenic protein, islet amyloid polypeptide, exhibited aggregation but not toxicity, suggesting that beta AP-induced neurotoxicity in vitro is not a nonspecific reaction to aggregated protein. The correlation between beta AP aggregation and neurotoxicity was also observed in long-term neuronal cultures but not in astrocyte cultures. These data are consistent with the hypothesis that beta-amyloid protein contributes to neurodegeneration in Alzheimer9s disease.

https://www.jneurosci.org/content/jneuro/13/4/1676.full.pdf

Neurodegeneration induced by beta-amyloid peptides in vitro: the role of peptide assembly state

The molecular mechanism responsible for the neurodegeneration in Alzheimer disease is not known; however, accumulating evidence suggests that beta-amyloid peptide (A beta P) contributes to this degeneration. We now report that synthetic A beta Ps trigger the degeneration of cultured neurons through activation of an apoptotic pathway. Neurons treated with A beta Ps exhibit morphological and biochemical characteristics of apoptosis, including membrane blebbing, compaction of nuclear chromatin, and internucleosomal DNA fragmentation. Aurintricarboxylic acid, an inhibitor of nucleases, prevents DNA fragmentation and delays cell death. Our in vitro results suggest that apoptosis may play a role in the neuronal loss associated with Alzheimer disease.

https://www.pnas.org/content/pnas/90/17/7951.full.pdf

Apoptosis is induced by beta-amyloid in cultured central nervous system neurons

The neurodegeneration of Alzheimer's disease has been theorized to be mediated, at least in part, by insoluble aggregates of beta-amyloid protein that are widely distributed in the form of plaques throughout brain regions affected by the disease. Previous studies by our laboratory and others have demonstrated that the neurotoxicity of beta-amyloid in vitro is dependent upon its spontaneous adoption of an aggregated structure. In this study, we report extensive structure-activity analyses of a series of peptides derived from both the proposed active fragment of beta-amyloid, beta 25-35, and the full-length protein, beta 1-42. We examine the effects of amino acid residue deletions and substitutions on the ability of beta-amyloid peptides to both form sedimentable aggregates and induce toxicity in cultured hippocampal neurons. We observe that significant levels of peptide aggregation are always associated with significant beta-amyloid-induced neurotoxicity. Further, both N- and C-terminal regions of beta 25-35 appear to contribute to these processes. In particular, significant disruption of peptide aggregation and toxicity result from alterations in the beta 33-35 region. In beta 1-42 peptides, aggregation disruption is evidenced by changes in both electrophoresis profiles and fibril morphology visualized at the light and electron microscope levels. Using circular dichroism analysis in a subset of peptides, we observed classic features of beta-sheet secondary structure in aggregating, toxic beta-amyloid peptides but not in nonaggregating, nontoxic beta-amyloid peptides. Together, these data further define the primary and secondary structures of beta-amyloid that are involved in its in vitro assembly into neurotoxic peptide aggregates and may underlie both its pathological deposition and subsequent degenerative effects in Alzheimer's disease.

Structure‐Activity Analyses of β‐Amyloid Peptides: Contributions of the β25–35 Region to Aggregation and Neurotoxicity

Amnesia induced in mice by centrally administered beta-amyloid peptides involves cholinergic dysfunction.

Animal models aim to replicate the symptoms, the lesions or the cause(s) of Alzheimer disease. Numerous mouse transgenic lines have now succeeded in partially reproducing its lesions: the extracellular deposits of A peptide and the intracellular accumulation of tau protein. Mutated human APP transgenes result in the deposition of A peptide, similar but not identical to the A peptide of human senile plaque. Amyloid angiopathy is common. Besides the deposition of A, axon dystrophy and alteration of dendrites have been observed. All of the mutations cause an increase in A 42 levels, except for the Arctic mutation, which alters the A sequence itself. Overexpressing wild- type APP alone (as in the murine models of human trisomy 21) causes no A deposition in most mouse lines. Doubly (APP £ mutated PS1) transgenic mice develop the lesions earlier. Transgenic mice in which BACE1 has been knocked out or overexpressed have been produced, as well as lines with altered expression of neprilysin, the main degrading enzyme of A. The APP transgenic mice have raised new questions concerning the mechanisms of neuro- nal loss, the accumulation of A in the cell body of the neu- rons, inXammation and gliosis, and the dendritic alterations. They have allowed some insight to be gained into the kinet- ics of the changes. The connection between the symptoms, the lesions and the increase in A oligomers has been found to be diYcult to unravel. NeuroWbrillary tangles are only found in mouse lines that overexpress mutated tau or human tau on a murine tau i/i background. A triply trans- genic model (mutated APP, PS1 and tau) recapitulates the alterations seen in AD but its physiological relevance may be discussed. A number of modulators of A or of tau accu- mulation have been tested. A transgenic model may be ana- lyzed at three levels at least (symptoms, lesions, cause of the disease), and a reading key is proposed to summarize this analysis.

/pdf/alzheimer-disease-models-and-human-neuropathology-4vzx82n5cs.pdf

Michael C. Merriman

Papers

Intracerebral /sB-amyloid(25–35) produces tissue damage: Is it neurotoxic?