23 May 2008. Two papers out this week focus on synapses, their contents and function, and how amyloid-β (Aβ) might play into both. One report uses a novel flow cytometry method to measure Aβ and phospho-tau proteins in synaptosomes prepared from human brain tissue, and shows that proteins often cohabit in synapses. The second paper brings together a new player (the polo-like kinase 2, Plk2) and a familiar one (Cdk5) in a new pathway that regulates synapse strength in a way that could be sensitive to Aβ.
In the first report, Jeffrey Fein, Karen Hoppens Gylys, and colleagues from the University of California at Los Angeles use a clever method to quantitate Aβ and phospho-tau in synaptic terminals from AD brain. The researchers prepared synaptosomes from postmortem brain tissue, which they then analyzed by flow cytometry after immunofluorescent labeling of Aβ or phospho-tau. Synaptosomes are spherical one-micron vesicles that form from synaptic ends and enclose the contents of synaptic terminals. The technique gets around the problems that synaptic terminals are hard to see in intact tissue by light microscopy, and that it is difficult to pin down the exact cellular location of proteins by immunofluorescence. The researchers had previously shown that the technique can detect elevated synaptic Aβ and cholesterol in AD brain (Gylys et al., 2004). The new report, published May 8 in the American Journal of Pathology, expands the method to also measure phospho-tau (Ser212/Thr214, as recognized by the AT100 antibody), and to examine several brain regions.
The results indicate that Aβ and phospho-tau indeed colocalize in synapses in AD brain. Nearly three-quarters of synaptosomes were positive for Aβ. One quarter of the Aβ-positive vesicles also stained with p-tau antibody, with the highest fraction in the entorhinal cortex, the brain region affected first by AD. The distributions of the two proteins are not identical—the investigators found the highest levels of synaptic Aβ in neocortex, the lowest in cerebellum, and intermediate levels in the hippocampus and entorhinal cortex. In contrast, tau levels were highest in hippocampus and entorhinal cortex, lower in neocortex, and undetectable in cerebellum.
“These results showing overlapping Aβ and tau pathology are consistent with a model in which both synaptic loss and dysfunction are linked to a synaptic amyloid cascade within the synaptic compartment,” the authors conclude.
Interestingly, the levels of synaptic Aβ measured by flow cytometry in different brain samples correlated well with levels of an Aβ hexamer on Western blots of the synaptosomes, suggesting that the synaptic Aβ may be an oligomeric form.
A second paper, which came out May 22 in Neuron, presents a new pathway for the regulation of synaptic strength that could be sensitive to Aβ and tau effects. The data, from the lab of Morgan Sheng at MIT, implicates the cyclin-dependent kinase 5 (Cdk5) in synaptic scaling, a mechanism by which neurons homeostatically adjust the strength of their synapses in response to very high or very low activity. The investigators find that synaptic scaling in response to high neuronal activity requires the induction of the Plk2 kinase, which then phosphorylates a scaffolding protein, SPAR. Subsequent SPAR degradation leads to a weakening of synaptic strength. The role of Cdk5 is to serve as a priming kinase that phosphorylates Plk2 recognition sequences on SPAR.
Cdk5 is familiar to AD researchers as a regulator of Aβ production (see ARF related news story and ARF related news story) and a tau kinase (see ARF related news story). The Cdk5 activator p25 has been reported to be elevated in AD brain and causes neurodegeneration in mice (see ARF related news story), and Cdk5 has been implicated in dendritic spine loss (see ARF related news story). There have also been reports that Aβ activates Cdk5 (Alvarez et al., 2001; Town et al., 2002), and Sheng and colleagues show that treatment of hippocampal cells with Aβ overnight induces the priming phosphorylation of SPAR, which they attribute to Cdk5 activation. The new results offer another potential pathway for Cdk5, and by association synaptic tau and Aβ, to affect synapse stability.—Pat McCaffrey.
Fein JA, Sokolow S, Miller CA, Vinters HV, Yang F, Cole GM, Gylys KH. Co-Localization of Amyloid beta and Tau Pathology in Alzheimer's Disease Synaptosomes. Am J Pathol. 2008 May 8. [Epub ahead of print] Abstract
Seeburg DP, Feliu-Mojer M, Gaiottino J, Pak DTS, Sheng M. Critical Role of Cdk5 and Polo-like Kinase 2 in Homeostatic Synaptic Plasticity during Elevated Activity. Neuron. 2008 May 22;58:571-583. Abstract