Small-molecule degraders reduce Aβ production through CAPRIN1-mediated lysosomal degradation of APP in Alzheimer’s iPSC-derived neurons

INTRODUCTION The amyloid cascade of amyloid β (Aβ) production from amyloid precursor protein (APP) plays a key role in the pathogenesis of Alzheimer’s disease (AD). However, disease-modifying therapies to disrupt this amyloid cascade remain an unmet challenge. METHODS Using neurons and organoids differentiated from induced pluripotent stem cells (iPSCs) derived from sporadic and familiar AD patients, we identified small-molecule degraders that reduce Aβ through screening of our library of cytoplasmic activation/proliferation-associate protein 1 (CAPRIN1)-targeted small-molecules. RESULTS The small-molecule degraders act as molecular glues that bind CAPRIN1 and APP and enhance the protein-protein interaction. Tracking CAPRIN1 and APP in neurons revealed that the degraders induce CAPRIN1-mediated APP degradation through the endosomal-lysosomal pathway and thus reduce Aβ peptide production in AD iPSC neurons and extracellular deposition in AD iPSC organoids. DISCUSSION The small-molecule degraders provide new therapeutics through targeted protein degradation of APP and disruption of the amyloid cascade in AD brain. HIGHTLIGHTS RESEARCH IN CONTEXT 1. Systematic review: Current literature indicates that targeted protein degradation is a rising strategy in drug discovery of small-molecule degraders that recruit disease-causing proteins to the proteasomal and lysosomal pathway for destruction. We have reported CAPRIN1-targeted small-molecule degraders and established CAPRIN1-targeted molecules library. 2. Interpretation: In screening of the library, we identified hit compound 0043 that degrades APP and reduces Aβ42 production in AD iPSC neurons. The structure-activity relationship (SAR) studies of 0043 analogs identified the more potent and drug-like lead compound 0152. The compounds act as molecular glues that bind CAPRIN1 and APP and enhance the protein-protein interaction. Tracking CAPRIN1 and APP through cellular organelles unveiled the mechanism of action of the degraders that induce CAPRIN1-mediated APP degradation through the endosomal-lysosomal pathway and thereby reduce Aβ peptide production and extracellular deposition in AD derived neurons and organoids. 3. Future direction: This study provides a proof-of-principle that small-molecule glues degrade APP and reduce Aβ using AD iPSC neuronal and organoid models. Further structure-activity relationship studies through chemical modifications of the compounds are necessary to identify more potent and blood brain barrier (BBB) permeable compounds for AD animal model studies and thereby preclinical development as the first-in-class drugs for treatment of AD patients. ### Competing Interest Statement Anita C. Bellail and Chunhai Hao are the co-founders of HB Therapeutics, Inc. and Degrome Therapeutics, Inc. Ho-Yin Lo is the founder and an employee of Synovel Laboratory. The U.S. provisional patent application 63/611,294 entitled [Compounds and methods to treat Alzheimer disease] was filed by Indiana University. Other authors declare no conflict of interest.