Single-Neuron Observations Reveal Alzheimer Stages

Alzheimer-related changes in the visual cortex have been observed at the single-cell level using two-photon calcium imaging. The changes could reveal distinct stages in Alzheimer’s disease with a specific order in time.

Alzheimer’s not only has devastating effects on memory and learning, but it can impair a person’s sense of smell or vision. Typically, these changes in sensory cognition show themselves behaviorally only when the disease is more advanced.

Microscopic images of a brain section (bottom: overview, top: enlarged view of the visual cortex), obtained from an AP23xPS45 Alzheimer mouse, in which the beta-amyloid plaques are labeled with Thioflavin-S. (Images: Konnerth Lab, TU Munich)

Now, research conducted at Technical University Munich (TUM) sheds light on what happens in the brain throughout the disease process, specifically to the part of the cerebral cortex responsible for integrating visual information. While studying a mouse model of Alzheimer’s disease, the neuroscientists discovered correlations between increases in both soluble and plaque-forming beta-amyloid — a protein implicated in the disease process of Alzheimer’s — and dysfunctional developments on several levels, including individual cortical neurons, sensory cognition, neuronal circuits and behavior.

Using two-photon calcium imaging, the researchers recorded both spontaneous and stimulated signaling activity in cortical neurons of living mice: transgenic mice carrying mutations that cause Alzheimer’s disease in humans, and wild-type mice as a control group. Observations of how neuronal signaling responded to a special kind of vision test — in which a simple grating pattern of light and dark bars moves in front of the mouse’s eye — enabled the scientists to characterize the visual circuit as being more or less “tuned” to specific orientations and directions of movement.

Schematic shows the experimental arrangement for in vivo two-photon calcium imaging of stimulation-evoked neuronal activity in anesthetized mice. At left, in vivo two-photon image of the visual cortex. The neurons are stained with the calcium indicator dye Oregon Green BAPTA-1 (green, OBG-1) and the astrocytes with sulforhodamine 101 (yellow, SR101). At right, visual stimuli were projected on a screen placed in front of the eye of the mouse.

“Like many Alzheimer’s patients, the diseased mice have impairments in their ability to discriminate visual objects,” said professor Arthur Konnerth, a Carl von Linde Senior Fellow of the TUM Institute for Advanced Study. “Our results provide important new insights on the cause that may underlie the impaired behavior, by identifying in the visual cortex a fraction of neurons with a strongly disturbed function.”

Within this group, they observed that there are two subsets of neurons, both of which are dysfunctional, but in completely different ways. One subset, thought to be the first neurons to degenerate, showed no activity at all; the other showed a pathologically high level of activity, rendering these neurons incapable of properly sensing objects in the mouse’s environment.

“While around half of the neurons in the visual cortex were disturbed in one way or the other, roughly half responded normally,” said Christine Grienberger, a doctoral candidate in Konnerth’s institute and first author of the paper. “That could have significant implications for future research in the field of Alzheimer’s disease, as our findings raise the question of whether future work only needs to target this population of neurons that are disturbed in their function.”

Table illustrates staged decline of the cortical circuit function on the level of the beta-amyloid load, the behavioral deficits and the visual processing.

The in vivo single-neuron experiments were carried out for three age groups, corresponding to different stages of this progressive, degenerative disease. The results were correlated with other measurements, including soluble beta-amyloid levels and the density of beta-amyloid plaques in the brain tissue. The researchers’ findings show for the first time a progressive decline of function in cortical circuits.

“An important conclusion from this study is that the Alzheimer’s disease-related changes on all levels — including behavior, cortical circuit dysfunction and the density of amyloid plaques in diseased brains — progress in parallel in a distinct temporal order,” Konnerth said. “In the future, the identification of such stages in patients may help researchers pinpoint stage-specific and effective therapies, with reduced levels of side effects.”

The study appeared in the April 10 issue of Nature Communications.

For more information, visit: www.tum.de