By Minji Kim, Alice Lu, and Rudy Tanzi.
Sam Gandy, Thomas Jefferson University, Philadelphia, reviewed the trafficking pathways of APP from the endoplasmic reticulum (ER) to the trans-Golgi network (TGN) to the plasma membrane (PM), including regulated ectodomain shedding by α-secretase. He discussed the search for phospho-state-sensitive modulators of ectodomain shedding (PMES) candidates, including munc13, munc18, PKD2wt, and PKD2KD, all of which boosted basal and regulated shedding. However, none of these candidates were found to regulate shedding in a phospho-state-sensitive fashion. Gandy then addressed the evidence that PS1 plays a role in trafficking of APP or other membrane proteins. In experiments using APP:furin chimeric mice, he showed that brain Aβ42/40+42 ratios were similar to ratios associated with FAD-mutant PS1, suggesting that altered trafficking of APP out of the TGN could underlie PS1-linked FAD. Similar to PS1 FAD mutants, distribution of an APP:furin chimera in CNS neurons in vivo was more restricted to the TGN than was that of APP:APP. Moreover, APP:furin expressed in CNS neurons generated more Aβ42/40+42 than APP:APP. Based on these and other findings, Gandy suggested that impaired TGN egress of APP may enhance the generation of Aβ42.
Roberto Malinow, Cold Spring Harbor Laboratory, Cold Spring Harbor, New York, reviewed the effects of APP and Aβ on synaptic function. He presented data showing that when synaptic activity is increased, processing of APP to Aβ and secretion of the peptide is increased. Aβ was able to depress synaptic function, thus forming a potential negative feedback loop. Malinow also showed that in hippocampal slice cultures, both Aβ-induced depression and plasticity-induced synaptic depression reduce dendritic spine density, and reduce numbers of synaptic NMDA and AMPA receptors. These events, which required calcineurin and p38-MapK, partially occluded metabotropic glutamate receptor-mediated long-term depression. Overall, these studies provided further support for a possible role of APP and Aβ in modulating synaptic activity.
Thomas Südhof, University of Texas Southwestern Medical Center in Dallas, described CSPα (cysteine-string protein-α) as a synaptic vesicle protein that contains a DNA-J domain typical for Hsp40-type co-chaperones. CSPα activates the ATPase activity of Hsc70, and co-assembles with Hsc70 and the small glutamine-rich tetratricopeptide repeat (TPR)-containing protein (SGT) into an enzymatically active chaperone. In mice, CSPα prevented presynaptic neurodegeneration and acted at least in part on SNARE proteins. Overexpression of α-synuclein suppressed neurodegeneration induced by deletion of CSPα, whereas endogenous α-synuclein protected against neurodegeneration (see ARF related news story). In its capacity as a neuroprotective molecule, α-synuclein did not appear to act as a chaperone, but bound lipid coats of synaptic vesicles and influenced the activity of SNARE proteins. In contrast, overexpression of human α-synuclein caused neurodegeneration, but by an unrelated mechanism, most likely involving oligomerization and aggregation.
In the short talk slated for this session, Constanze Reinhard, Center for Human Genetics, KU Leuven, presented data from her search for novel receptors for secreted forms of APP (sAPP). By establishing a system to evaluate sAPP binding properties, she showed that sAPP binding occurs via two different mechanisms: a heparan sulfate-dependent interaction and a protein-protein interaction. In the neuroblastoma cell lines, N2A and B10-3, sAPP preferentially binds through a protein-protein mediated interaction.