Alexander Wallace

Introduction: Alzheimer’s Disease (AD) is a fatal, progressive neurodegenerative disease that affects over 27 million people worldwide and is characterized by gradual, aggressive cognitive deterioration caused by deposition of amyloid-beta (Ab) after cleavage of amyloid precursor protein (APP) by beta(b)-secretase and gamma(g)-secretase^{1, 2}. Treatment is limited to acetylcholinesterase-inhibitors and NMDA-inhibitors, which temporarily mitigate cognitive impairment, and a single, recently-approved anti-Ab monoclonal antibody³. New research revisiting g-secretase has demonstrated its potential as a therapeutic target of AD. Methods: HEK293 cells and human neurons were incubated with APP and g-secretase components, lysed, and immunoassayed for Ab fragments; similar procedures were performed with roburic acid (RA) added during incubation⁴. Ab fragments were quantified following alanine mutagenesis of all residues of hydrophilic loop 1 (HL-1) in HEK293 cells and again after incubation with acidic and heterocyclic g-secretase modulators (GSMs)⁴. Ab fragments were also assayed in HEK293 cells after g-secretase activating protein (GSAP) knockout and after reintroduction⁶. X-ray crystallography was performed on g-secretase as bound to g-secretase inhibitors (GSIs) semagacestat, avagacestat, and L-685,458 and to GSM E2012 to evaluate their binding locations⁷. Ab fragments were quantified after oral administration of a novel GSM⁸. Results: Western blotting and Ab fragment quantification of HEK293 and human neurons incubated with APP and g-secretase components demonstrated that a high molecular weight (HMW) complex of b-secretase and g-secretase exists and accounts for most of the Ab production in humans⁴. Roburic acid-incubated cells significantly reduced production of Ab fragments and impaired HMW complex formation⁴. The alanine mutant at residue 106 in HL-1 altered g-secretase processivity to form more of the smaller, less-pathogenic Ab fragments⁵. Furthermore, heterocyclic GSMs did not function in the 106A mutant cells⁵. The GSAP knockout experiment determined that GSAP knockout significantly reduces g-secretase activity while restoration of GSAP restores it⁶. X-ray crystallography experiments demonstrated that semagacestat and avagaestat bind at g-secretase’s active site, as does L-685,458; heterocyclic GSM E2012 closely associates with residue 106 in HL-1⁷. Single doses of a novel GSM at dosages of 5 mg/kg and 10 mg/kg in mice and at various dosages in non-human primates yielded significant reductions in longer, more-pathogenic Ab fragments⁸. Conclusions: g-secretase is still promising for therapeutic purposes, with many key aspects of its regulation representing possible drug targets, such as: disruption of the HMW complex, GSMs targeting HL-1, inhibition of GSAP, and chemical modification of previously-failed GSIs (semagacestat and avagacestat). Lastly, the novel GSM in preclinical development shows great promise in reducing Ab but requires further characterization.

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