M.R. Sierks, S. Boddapati, S. Kasturirangan

Deposition of misfolded amyloid beta (abeta) is one of the important neuropathological characteristics of Alzheimer’s disease (AD). Abeta is produced from the Amyloid Precursor Protein by the sequential activity of two enzymes, β–secretase and γ-secretase. A third protease, α-secretase, cleaves APP in the middle of the abeta sequence precluding formation of Abeta. The levels of abeta generated from APP can therefore be controlled by tailoring the activity of these proteases toward APP. Inhibiting β–secretase activity is a promising approach to decrease amyloidogenic processing of APP to Abeta, however β–secretase has other preferred substrates so inhibiting the enzyme itself may have unwanted side effects. An alternative approach is to block β–secretase activity using antibodies that bind to the APP substrate at the β–secretase cleavage site. We isolated and characterized a single chain variable domain antibody fragment (scFv) that inhibit the activity of β-secretase by binding APP (iBSEC1). The iBSEC1 scFv potently reduced abeta levels in cell models of AD and reduced toxicity. We also generated a proteolytic scFv engineered (Asec1A) to cleave at the α-secretase site of APP to promote non-amyloidogenic processing of APP by increasing α-secretase activity towards APP. The Asec1A proteolytic scFv reduced aggregation and toxicity of abeta and also decreased APP levels and toxicity in cell models of AD. While the iBSEC1 and Asec1A scFvs both by themselves work effectively to reduce abeta generation from APP, the two scFvs can be combined into a bispecific recombinant antibody DIA10D. When expressed in cell models of AD, the DIA10D diabody potently reduces levels of extracellular abeta and increases the levels of the neuroprotective APP product, sAPPα. By simultaneously inhibiting β-secretase processing and increasing α-secretase processing of APP, we can direct processing of APP towards the neuroprotective non-amyloidogenic pathway.

Keywords (Optional): 
amyloid precursor protein
BACE inhibitor
proteolytic antibody