The Amyloid Hypothesis is a proposed model describing the development of Alzheimer’s disease. The deposition of excess amyloid- β peptide leads to the formation of amyloid plaques which then go on to form neurofibrillary tangles. These tangles are further composed of hyperphosphorylated Tau. However, there has been doubt surrounding this hypothesis as some mouse models with mutations in Familial Alzheimer’s disease (FAD) genes do not show the same development of the disease as humans, forming amyloid plaques but not forming neurofibrillary tangles. Studies using human neurons from Alzheimer’s patients have also shown that these cells display increases in toxic amyloid-β species and phosphorylated Tau but without the formation of amyloid plaques or neurofibrillary tangles.
This month a group in the US have published an elegant study using a 3D cell culture system to recapitulate the development of ‘Alzheimer’s disease in a dish’, providing evidence that the 30 year old Amyloid Hypothesis may well be correct and raising hopes for the future of Alzheimer’s research.
The group overexpressed human beta-amyloid precursor protein (APP) and presenilin (PSEN1) constructs with FAD mutations in human neural progenitor cells (ReN cells). These cells then differentiated into neuronal and glial cells within three weeks and exhibited an increase in neuronal marker genes as well as increased levels of Tau and amyloid-β isoforms. They then transferred these cells to a 3D culture system using BD Matrigel containing high levels of brain extracellular matrix proteins. They observed that this 3D culture method promoted more neuronal and glial differentiation than equivalent 2D cultures, together with increased Tau isoform expression.
After a further 2-6 weeks of differentiation, an increase in extracellular amyloid-β deposits was observed in the FAD ReN cells compared to the controls. The level of deposition could be decreased by treatment with β-secretase inhibitor IV or γ- secretase inhibitors DAPT and SGSM41. The FAD ReN cells also exhibited accumulation of insoluble amyloid-β aggregates, which again could be decreased by treatment with γ- secretase inhibitors.
Amyloid-β deposits in control (ReN-G) and FAD ReN (ReN-mGAP)
The group also analysed levels of phosphorylated Tau in the FAD ReN cells and detected a large increase in levels of both phosphorylated and total Tau. The level of phosphorylated Tau in the FAD ReN cells could also be decreased by treatment with β- or γ- secretase inhibitors, suggesting that phosphorylated Tau accumulation in these cells is a consequence of amyloid-β accumulation. They further observed phosphorylated Tau aggregates and filamentous structures that were strikingly similar to those structures observed in Alzheimer’s patient brains. Treatment with GSK3β inhibitors 1-azakenpaullone and SB415286 reduced levels of phosphorylated Tau without affecting total Tau or amyloid-β levels, suggesting a role for GSK3β in tauopathy downstream of deposition of amyloid-β.
This important work has not only provided experimental evidence supporting the Amyloid Hypothesis of Alzheimer’s disease but also generated a model system that can be used for studying the mechanisms behind the pathology of the disease. The 3D culture system could also be used for screening of drugs and could potentially be modified to develop models for other neurodegenerative disorders.
Choi SH, Kim YH, Hebisch M, Sliwinski C, Lee S, D’Avanzo C, Chen H, Hooli B, Asselin C, Muffat J, Klee JB, Zhang C, Wainger BJ, Peitz M, Kovacs DM, Woolf CJ, Wagner SL, Tanzi RE, Kim DY., A three-dimensional human neural cell culture model of Alzheimer’s disease., Nature. 2014 Oct 12. doi: 10.1038/nature13800. [Epub ahead of print]