Glycogen synthase kinase-3beta regulates presenilin 1 C-terminal fragment levels

The majority of familial Alzheimer's disease cases have been attributed to mutations in the presenilin 1 (PS1) gene. PS1 is synthesized as an inactive holoprotein that undergoes endoproteolytic processing to generate a functional N- and C-terminal heterodimer (NTF and CTF, respectively). We identified a single residue in PS1, Ser(397), which regulates the CTF levels in a population of dimer that has a rapid turnover. This residue is part of a highly conserved glycogen synthase kinase-3beta (GSK-3beta) consensus phosphorylation site within the loop domain of PS1. Site-directed mutagenesis at the Ser(397) position increased levels of PS1 CTF but not NTF or holoprotein. Similar increases in only CTF levels were seen when cells expressing wild type PS1 were treated with lithium chloride, an inhibitor of GSK-3beta. Both wild type and PS1 S397A CTF displayed a biphasic turnover, reflecting rapidly degraded and stable populations. Rapid turnover was delayed for mutant PS1 S397A, causing increased CTF. These data demonstrate that PS1 NTF.CTF endoproteolytic fragments are generated in excess, that phosphorylation at Ser(397) by GSK-3beta regulates the discard of excess CTF, and that the disposal of surplus NTF is mediated by an independent mechanism. Overall, the results indicate that production of active NTF.CTF dimer is more complex than limited endoproteolysis of PS1 holoprotein and instead involves additional regulatory events.