Poster Session - Abstract # 18
Familial Alzheimer’s Disease Mutations Stabilize Stalled Complexes of γ-secretase Bound to Substrate and Trigger Synaptic Loss Independent of Aβ42
Sujan Devkota1, Rui Zhou2, Vaishnavi Nagarajan1, Masato Maesako3, Shweta R. Malvankar1, Hung Do4, Sanjay Bhattarai1, Anita Saraf4, Yinglong Miao5,6, Brian D. Ackley6, Yigong Shi2, and Michael S. Wolfe1
1Department of Medicinal Chemistry, University of Kansas; 2Beijing Advanced Innovation Center for Structural Biology, Beijing Frontier Research Center for Biological Structure, Tsinghua-Peking Joint Center for Life Sciences, School of Life Sciences, Tsinghua University; 3Alzheimer Research Unit, Mass General Institute for Neurodegenerative Disease, Massachusetts General Hospital, Harvard Medical School; 4Mass Spectrometry and Analytical Proteomic Laboratory, University of Kansas; 5Center for Computational Biology, University of Kansas; 6Department of Molecular Biosciences, University of Kansas
Alzheimer’s disease (AD) is a neurodegenerative disorder that causes dementia. AD is characterized pathologically by cerebral deposition of 42-residue variant of amyloid β-peptide (Aβ42), produced from amyloid precursor protein (APP) by β- and γ-secretases. The γ-secretase carries out processive proteolysis within the transmembrane domain (TMD) of APP substrate to generate Aβ40 or Aβ42 along two different pathways: Aβ49→Aβ46 →Aβ43→Aβ40→Aβ37 and Aβ48→Aβ45 →Aβ42→Aβ38. Although mutations in APP and presenilin, the catalytic component of γ-secretase, cause familial Alzheimer’s disease (FAD), Aβ42 pathogenicity has not been clearly established. In this study we show that FAD-mutant γ-secretases are deficient in early proteolytic steps of APP processing and not later steps that produce secreted Aβ products when compared to wild-type (WT) enzyme. A substrate-based TMD mimetic traps the enzyme in its transition state of substrate cleavage and validates a molecular dynamics model for the active enzyme. Dynamic simulations and live cell imaging by fluorescence lifetime imaging microscopy suggests that the FAD-mutant γ-secretases bound to APP substrate are less flexible and more stable compared to WT enzyme-substrate complexes. A C. elegans transgenic lines expressing FAD-mutant APP substrate and/or Presenilin-1 (PSEN1) showed shorter lifespan and synaptic loss compared to WT transgenic lines. FAD-mutant APP substrate required PSEN1 for the neurodegenerative phenotype, but FAD-mutant PSEN1 led to neurodegeneration even in the absence of coexpressed APP substrate. Addition of a mutation that blocks Aβ42 production also showed synaptic loss and reduced lifespan. Taken together, these findings show that FAD mutations can cause neurodegeneration independently of Aβ42 and suggest that stalled complexes of γ-secretase and substrate may trigger the disease.