How dominant mutations in presenilin (PSEN) cause early-onset familial Alzheimer’s disease (FAD) has been debated since the discovery of such mutations 20 years ago. A study by Szaruga et al. in this issue of JEM now appears to provide a definitive answer.
Presenilin is the catalytic subunit of γ-secretase, a protease that cuts the transmembrane domain of the amyloid precursor protein (APP) to produce the C terminus of the amyloid β-peptide (Aβ) that notoriously deposits in the Alzheimer brain. Some argue that reduction of presenilin’s proteolytic activity (i.e., a loss-of-function effect) is responsible for the neurodegeneration caused by FAD mutations. Others have shown that some mutations do not reduce proteolytic activity, but all increase the proportion of aggregation-prone 42-residue Aβ (Aβ42) to 40-residue Aβ (Aβ40; i.e., a gain of toxic function). Further complicating matters, γ-secretase initially cuts APP substrate via an endopeptidase activity to produce Aβ48 and Aβ49 and release the corresponding APP intracellular domain (AICD). These long Aβ peptides are then sequentially trimmed via a carboxypeptidase function of γ-secretase along two primary pathways: Aβ49 → Aβ46 → Aβ43 → Aβ40 and Aβ48 → Aβ45 → Aβ42 → Aβ38.
To address the loss- versus gain-of-function question, Szaruga et al. examined γ-secretase proteolytic activity in samples from post-mortem human brains from 24 FAD mutation carriers, covering nine different PSEN mutations. The samples contained endogenous human γ-secretase complexes and—importantly—both wild-type and PSEN mutant complexes. Under these natural conditions associated with the human disease state, the production of AICD—a measure of γ-secretase endoprotease activity—was not significantly different from that seen in control non-AD brains. Thus, the presence of the wild-type PSEN allele apparently compensates for any loss of endoproteolytic activity from the mutant allele. In contrast, clear reduction of carboxypeptidase activity—as measured by the ratio of Aβ38 from its precursor Aβ42—was seen for every mutation.
These findings have implications for the mechanism of Alzheimer pathogenesis and for drug discovery. In considering γ-secretase as a therapeutic target, one should first know what specific functional alterations in the enzyme lead to disease, and that appears to be decreased carboxypeptidase activity. Therefore, a search for stimulators of this activity would make sense. Such compounds have already been identified, although they appear to stimulate only the Aβ42 → Aβ38 step, insufficient if other long Aβ peptides are augmented in Alzheimer’s and play pathogenic roles. As is so often the case, answering one key question leads to another.