Imaging structure and function in the living brain

Transgenic mouse models are available that allow in vivo observation of the development of Alzheimer pathology. Multiphoton microscopy is an imaging technique that provides high-resolution imaging of the mouse brain with limited phototoxicity. Using this technique, we are able to image both structural and functional markers in the living brain over time. This allows the direct monitoring of progression of the disease, and provides an ideal detection platform for the evaluation of therapeutics aimed at reversing or preventing the deleterious outcomes. We have recently adapted a variant of the green fluorescent protein that is sensitive to redox potential to evaluate the role of oxidative stress in the pathogenesis of neuronal degeneration and death in the mouse models of Alzheimer's disease. This probe allowed us to measure directly the cellular redox state in neurons using AAV-mediated expression of the redox-sensitive GFP (roGFP). Using this GFP variant, we could image both structural and functional alterations of neurons in the brain longitudinally. In the cortex of APP/PS1 transgenic mice, we observed that cellular oxidative stress occurs in a small fraction of individual neuronal soma that immediately precedes neuronal death in living mice. The number of neurons that die is very small, and likely undetectable with standard stereology approaches in tissue sections, however, the functional imaging approach easily identified these at risk neurons. Furthermore, we identified Aß plaques as the focal source of this effect, generating oxidative stress in surrounding neurites that propagated into large brain areas over time. We present the time course of plaque deposition, development of intracellular oxidative stress, and subsequent neuronal death. These results demonstrate that oxidative stress plays a significant role in the brain; linking amyloid deposits to neurodegeneration. Future studies will be aimed at intervening along these pathways to prevent loss of cells, which must ultimately lead to neural network dysfunction. We anticipate that these studies will help guide the development of therapeutic agents for Alzheimer's disease.