A C. elegans model of familial Alzheimer's disease shows age-dependent synaptic degeneration independent of amyloid β-peptide
The membrane-embedded {gamma}-secretase complex is involved in the intramembrane cleavage of [~] 150 substrates. Cleavage of amyloid precursor protein (APP)-derived substrate C99 generates 38-43-residue secreted amyloid {beta}-peptides (A{beta}), with the aggregation-prone 42-residue form (A{beta}42) particularly implicated in the pathogenesis of Alzheimers Disease (AD). However, whether A{beta}42 is the primary driver of neurodegeneration in AD remains unclear. Dominant mutations in APP or presenilin--the catalytic component of {gamma}-secretase--cause early-onset familial AD (FAD) and reduce one or more steps in the multi-step processive proteolysis of C99 to A{beta} peptides, apparently through stabilization of {gamma}-secretase enzyme-substrate (E-S) complexes. To investigate mechanisms of neurodegeneration in FAD, we developed new C. elegans models co-expressing wild-type or FAD-mutant C99 substrate and presenilin-1 (PSEN1) variants in neurons, allowing intramembrane processing of C99 to A{beta} in vivo. We demonstrate that while FAD-mutation of either C99 or PSEN1 leads to age-dependent synaptic loss, proteolytically inactive PSEN1 did not. Designed mutations that allow stable E-S complex formation without A{beta}42 or A{beta} production likewise result in synaptic degeneration. Moreover, replacement of C99 with variants of a Notch1-based substrate revealed that disrupted processing of another {gamma}-secretase substrate can similarly lead to synaptic degeneration. These results support a model in which synaptic loss can be triggered by toxic, stalled {gamma}-secretase E-S complexes in the absence of A{beta} production and not by simple loss of proteolytic function. This new C. elegans system provides a powerful platform to study the role of dysfunctional {gamma}-secretase substrate processing in FAD pathogenesis. SIGNIFICANCEDominantly inherited familial Alzheimers disease (FAD) is caused by mutations in the enzyme or substrate that produces amyloid {beta}-peptides (A{beta}). These mutations alter enzyme processing of substrate to A{beta} and can skew production of A{beta} to aggregation-prone forms that deposit as pathological plaques. Nevertheless, whether A{beta} is the primary driver of AD remains unresolved. Recent evidence supports a model in which loss of neuronal connections in FAD is due to the stalled enzymatic process of A{beta} production, rather than the A{beta} products. Using the roundworm C. elegans as a genetic model system, we show here that the stalled process itself, rather than A{beta} or reduced enzyme activity, can trigger loss of neuronal connections with age in this simple model animal.