Alzheimer's disease (AD) is a neurodegenerative disorder that effects approximately 4.5 million Americans over the age of 65—a statistic that is set to triple by 2050. The widely accepted amyloid hypothesis proposes that the disease is caused by an accumulation of aggregating amyloid-b (Ab) protein, which eventually forms insoluble fibrils that deposit in the brain. This theory is supported by several genetic mutations linked to AD, which increase the aggregation potential of the protein. Furthermore, experimental studies show that aggregated Ab is toxic in neuronal cultures, while its monomeric form causes no detrimental effects (Tickler et al. 2005). Unfortunately, these toxic aggregates are not easily detected in patients. Differentiation between toxic aggregates and non-toxic, monomeric Ab is not trivial, and total Ab concentrations observed in vivo fall within the high picomolar to low nanomolar range (Scheuner 1996), with aggregates present at much lower concentrations. Effective therapeutic treatments will require administration early in the condition's onset; thus, early diagnosis, potentially via aggregate detection, is a critical next step in the development of therapeutic strategies for the disease. In this study, a quartz crystal microbalance (QCM) is developed as a selective biosensor for Ab aggregates based on the principle that only aggregated Ab, not the protein's monomeric form, is capable of short term growth. QCM utilizes the piezoelectric effect in quartz crystals to detect changes in bound elastic mass as a variation in the frequency of oscillation. An avidin monolayer is deposited onto the gold surface of a crystal electrode via well established amino-coupling chemistry (Jung et al. 2006), upon which biotinylated Ab monomer is bound. This surface selectively recognizes aggregated forms of Ab in the nanomolar range, whereas monomer-monomer interactions are virtually non-existent in the system. Subsequent exposure to monomer will initiate aggregate growth via aggregate-monomer interactions to generate an amplified signal for detection of low concentrations of aggregates that may be small in size. This QCM sensor successfully detected growth when nanomolar concentrations of aggregate were exposed to the crystal surface. Further quantification of aggregate growth rates revealed a first order dependence on monomer concentration. Furthermore, aggregate growth rates were found to increase in the presence of increasing ionic strength and decreasing pH. These parameters will allow for future optimization of the biosensor, as well as provide a better understanding of the aggregation process. Future work will explore the ability of the QCM sensor to selectively detect Ab aggregates in plasma and cerebral spinal fluid isolated from AD patients. Successful implementation of this technique could provide an effective means for the early diagnosis of AD.

Jung, H., Kim, J., Park, J., Lee, S., Lee, H., Kuboi, R., and Kawai, T. (2006). "Atomic force microscopy observation of highly arrayed phospholipid bilayer vesicle on a gold surface." Journal of Bioscience and Bioengineering, 102(1), 28-33.

Scheuner, D. E., C.; Jensen, M.; Song, X.; Citron, M.; Suzuki, N.; Bird, T. D.; Hardy, J.; Hutton, M.; et al. (1996). "Secreted amyloid &Beta -protein similar to that in the senile plaques of Alzheimer's disease is increased in vivo by the presenilin 1 and 2 and APP mutations linked to familial Alzheimer's disease." Nature Medicine, 2, 864-870.

Tickler, A. K., Wade, J. D., and Separovic, F. (2005). "The Role of Amyloid-beta Peptides in Alzheimer's Disease." Protein & Peptide Letters, 12(6), 513-519.