Allosteric modulation of PS1/gamma-secretase conformation correlates with amyloid beta(42/40) ratio

BACKGROUND

Presenilin 1(PS1) is the catalytic subunit of gamma-secretase, the enzyme responsible for the Abeta C-terminal cleavage site, which results in the production of Abeta peptides of various lengths. Production of longer forms of the Abeta peptide occur in patients with autosomal dominant Alzheimer disease (AD) due to mutations in presenilin. Many modulators of gamma-secretase function have been described. We hypothesize that these modulators act by a common mechanism by allosterically modifying the structure of presenilin.

METHODOLOGY/PRINCIPAL FINDINGS

To test this hypothesis we generated a genetically encoded GFP-PS1-RFP (G-PS1-R) FRET probe that allows monitoring of the conformation of the PS1 molecule in its native environment in live cells. We show that G-PS1-R can be incorporated into the gamma-secretase complex, reconstituting its activity in PS1/2 deficient cells. Using Förster resonance energy transfer (FRET)-based approaches we show that various pharmacological and genetic manipulations that target either gamma-secretase components (PS1, Pen2, Aph1) or gamma-secretase substrate (amyloid precursor protein, APP) and are known to change Abeta(42) production are associated with a consistent conformational change in PS1.

CONCLUSIONS/SIGNIFICANCE

These results strongly support the hypothesis that allosteric changes in PS1 conformation underlie changes in the Abeta(42/40) ratio. Direct measurement of physiological and pathological changes in the conformation of PS1/gamma-secretase may provide insight into molecular mechanism of Abeta(42) generation, which could be exploited therapeutically.

Control of Peripheral Nerve Myelination by the -Secretase BACE1

Although BACE1 (beta-site amyloid precursor protein-cleaving enzyme 1) is essential for the generation of amyloid-b peptide in Alzheimer's disease, its physiological function is unclear. We found that very high levels of BACE1 were expressed at time points when peripheral nerves become myelinated. Deficiency of BACE1 resulted in the accumulation of unprocessed neuregulin 1 (NRG1), an axonally expressed factor required for glial cell development and myelination. BACE1-/- mice displayed hypomyelination of peripheral nerves and aberrant axonal segregation of small-diameter afferent fibers, very similar to that seen in mice with mutations in type III NRG1 or Schwann cell-specific ErbB2 knockouts. Thus, BACE1 is required for myelination and correct bundling of axons by Schwann cells, probably through processing of type III NRG1.