9 October 2009. One look at neurons surrounding amyloid plaques shows they are not normal: Dystrophic neurites, aberrant axons, and neuroinflammation are visible signs that all is not well. However, do outward appearances translate to functional problems inside cells in vivo?
Part of the obstacle to answering that question has been a lack of methods for detecting activity in dendrites or dendrite segments in living tissue. Now, Bradley Hyman and colleagues at Massachusetts General Hospital in Charlestown have developed a genetic marker that reports dendritic activation in vivo in mouse brain neurons. In a paper published in the October 7 Journal of Neuroscience, they use the method to show that dendrites that are close to plaques have lower activity, and are unresponsive to stimulation by environmental enrichment. Neurons that are more distant from plaques are less affected. The results suggest that plaques are foci of neuropathology, and could precipitate system-wide dysfunction interfering with the integration of neuronal responses.
In developing a method for measuring neuronal function in dendritic segments, first author Melanie Meyer-Luehmann took advantage of the activity-dependent transport and local translation of mRNAs that occurs in dendrites. By attaching the regulatory regions of one such mRNA for the calcium/calmodulin-dependent kinase IIα, CAMKIIα, to the coding sequence for a green fluorescent protein (GFP), she designed a visual readout of local dendritic translation as a proxy for neuronal activation. In cultured cells, treatment with BDNF (which stimulates protein synthesis in dendrites) resulted in a subtle but significant increase in GFP fluorescence in both cell bodies and dendrites within 30 minutes.
Next, Meyer-Luehmann and colleagues established that the reporter could be used to give an in vivo snapshot of neuronal activity. They used adenovirus to transfect mouse brain somatosensory cortex with a similar CAMKIIα construct expressing the brighter yellow fluorescent protein (YFP), Venus, and then treated the transduced mice to a play session with new toys. They found that even a brief 30-minute environmental enrichment was enough to boost YFP production as detected by immunofluorescence on postmortem brain slices.
To ask what effect plaques had on surrounding neurons, they repeated that experiment in an APP/PS1 transgenic mouse model of Alzheimer disease. Overall, they found that the basal expression of the reporter protein was decreased compared to the wild-type mice, and the greatest diminution was seen closest to plaques. In neurons near plaques, YFP expression did not increase after environmental enrichment, even though the animals explored the novel toys just like wild-type mice. Neurons farther away (100 μm or more) showed higher YFP expression, and their cell bodies, but not their dendrites, registered an effect after environmental enrichment.
The impact of distance was dramatically illustrated when the researchers looked at different segments of the same dendrite. The portion nearest the plaque was not nearly as bright as segments on either side, and all the signals were unchanged after environmental enrichment.
“We conclude that individual plaques serve as focal pathophysiological lesions with a wide-ranging impact on nearby and distant neuronal systems,” the authors write. Just why plaques have this effect is unclear, but one possibility is a nearby elevation of soluble amyloid-β species, which the same researchers previously showed in association with a proximity-dependent loss of dendritic spines near plaques (see ARF related news story on Koffie et al., 2009).
In their view, numerous plaques would impair overall system functioning by “altering dendritic integration of information in a distributed manner throughout the cortex, preventing the activation of otherwise intact neuronal structures.” This could explain why the pattern of changes in brain function seen on fMRI correlates with the distribution of amyloid plaque deposition, they write (see ARF related news story on Buckner et al., 2009).—Pat McCaffrey.
Meyer-Luehmann M, Mielke M, Spires-Jones TL, Stoothoff W, Jones P, Bacskai BJ, Hyman BT. A reporter of local dendritic translocation shows plaque-related loss of neural system function in APP-transgenic mice. J. Neurosci. 2009 October 7; 29(40):12636-12640. Abstract