Dysfunction of -secretase has been linked to early-onset Alzheimers disease; however, the underlying molecular mechanisms are unknown. Inoue et al. have now shown that -secretase is localized to synaptic membranes, where it cleaves ephrin receptor A4 (EPHA4) on synaptic activation and triggers the formation of dendritic spines.
To examine the physiological function of -secretase, the authors first investigated its localization in neurons. They observed that -secretase colocalized with synaptic markers in hippocampal sections and cultured rat hippocampal neurons. They went on to show that compound E, an inhibitor of -secretase, reduced the density of dendritic spines. These results indi-cated that -secretase is important for the formation and maintenance of dendritic spines.
EPHA4 is a -secretase substrate and is localized in the postsynaptic membrane. A series of biochemical
studies in HEK293 cells expressing EPHA4 revealed that it is cleaved by -secretase to yield a cytosolic intracellular domain (ICD).
The authors demonstrated that, in neuronal cultures, treatments that activated glutamate receptors and therefore stimulated synaptic activity resulted in the generation of the ICD, and that this process was inhibited by compound E. In order to investigate the functional role of the ICD, the authors expressed this fragment in primary cultured hippocampal neurons. These neurons had a higher spine density than control cells and this was unaffected by treatment with compound E. Conversely, knockdown of EPHA4 using RNA interference decreased the number of spines. These results suggest that EPHA4 is a substrate of -secretase and that the EPHA4 ICD regulates spine formation.
EPHA4 is known to regulate the organization of the actin cytoskeleton through Rho family proteins, such
as RAC1. In neurons expressing the EPHA4 ICD, the enhanced forma-tion of dendritic spines was abolished when dominant-negative mutant RAC1N17 was co-expressed, indicat-ing that EPHA4-induced dendritic spine formation is mediated through activated RAC1.
Finally, the authors investigated mutations in -secretase that are linked to familial Alzheimers disease and showed that these mutations generated lower amounts of ICD than controls.
This study links mutations in -secretase with deficits in dendritic spine formation and suggests that the loss of dendritic spines might contribute to Alzheimers disease pathology.
ORIGINAL RESEARCH PAPER Inoue, E. et al. Synaptic activity prompts -secretase-mediated cleavage of EphA4 and dendritic spine formation. J. Cell Biol. 185, 551564 (2009)
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NATuRE REvIEws | NeuroscieNce volumE 10 | july 2009
Nature Reviews Neuroscience | AoP, published online 10 june 2009; doi:10.1038/nrn2673
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