The Molecular Imaging group focuses on the visualization of Alzheimer’s disease (AD) related changes in the brain of mouse models using in vivo multiphoton microscopy. In vivo imaging of individual AD lesions is essential for a better understanding of their functional impact, enhancing also the translational interpretation of amyloid imaging in humans. Moreover, in vivo monitoring of AD lesions and associated gliosis as well as neurodegeneration provides a powerful tool for evaluating in vivo therapeutic preclinical strategies.
1. To investigate the development of amyloid lesions and associated pathologies in vivo in the mouse brain.
2. To characterize novel amyloid-binding dyes with respect to their staining properties of different amyloid types and structural variances thereof, in vivo.
3. To study in vivo microglial behavior in the living brain, both in the context of normal aging and in various mouse models showing neurodegeneration.
To this end, we use a variety of transgenic mouse models of cerebral amyloidoses in combination with endogenously fluorophore-labeled microglia and/or neuronal subpopulations. In vivo examination is performed using multiphoton microscopy combined with a set of advanced imaging tools for stereological and 5D analysis of the morphological datasets.
We have used multiphoton in vivo imaging to study Aβ plaque formation in the brains of APPPS1 transgenic mice. By using a newly designed head fixation system (Hefendehl et al., J Neurosci Methods 2012) with automatic rapid repositioning to previously marked areas, we have successfully identified newly appearing amyloid deposits and tracked single amyloid plaques for up to 6 months. The results revealed that newly formed amyloid plaques develop in the APPPS1 transgenic mouse model at an estimated rate of 35 per mm3 of neocortical volume per week at the age of 4-5 months. At later time-points, i.e. with increasing cerebral β-amyloidosis, the number of newly formed plaques declines in comparison to the early phases of amyloid deposition. Both newly formed and existing plaques grow with a similar weekly increase of 0.3 μm in radius over the entire imaging period (Hefendehl et al., J Neuroscience 2011).
These results suggest that amyloid plaque formation is a spontaneous process that is likely dependent on the local concentration of Aβ, which in turn is in dynamic equilibrium with the insoluble Aβ associated with the amyloid plaques. Once aggregation has started, microglia move toward the amyloid and adopt a phagocytic morphology, but they appear to be unsuccessful in clearing the amyloid (Bolmont et al., J Neuroscience 2008). Independent of amyloid deposits, we recently showed that microglial cells exhibit an age-related loss of homogeneous tissue distribution, and that the movement of microglial somata increases in aged mice. However, in response to tissue injury the microglial response is age-dependently diminished (Hefendehl et al., Aging Cell, 2013).
Although various amyloids share a common three-dimensional structure with similar chemical and biophysical properties, they can exhibit conformational variations that cannot be distinguished using classical amyloid-binding dyes. In a recent study, we demonstrated that a novel class of amyloid-binding dyes (LCOs) cross the BBB and stain all major amyloid lesions in the brain (amyloid plaques, vascular amyloid and tau inclusions). Furthermore, because these new dyes exhibit a conformation-dependent spectral shift of their emitted fluorescence, they allow for spectral discrimination of structurally different amyloid lesions upon two-photon excitation (Wegenast-Braun et al., American Journal of Pathology, 2012). Current studies are assessing whether LCOs are also effective in distinguishing structural variations within the same lesion, e.g. due to changes in amyloid composition/structure as a function of disease progression. The effective separation of amyloid (sub)types may provide further insight into the linkage of distinct conformational amyloid structures to in vivo malfunctions.
A solid knowledge of the molecular characteristics and dynamics of cerebral amyloidosis is essential for the understanding of AD and other neurodegenerative disorders and is also critical for the interpretation of amyloid imaging in AD clinical studies. Furthermore, illuminating the role of microglia, the brain’s main immune cells capable of phagocytosis, could reveal new targets for therapeutic intervention.
Hefendehl JK, Neher JJ, Sühs RB, Kohsaka S, Skodras A, Jucker M (2013) Homeostatic and injury-induced microglia behavior in the aging brain. Aging Cell (Epub ahead of print) (Abstract)
Wegenast-Braun BM, Skodras A, Bayraktar G, Mahler J, Fritschi SK, Klingstedt T, Mason JJ, Hammarstrom P, Nilsson KP, Liebig C, Jucker M (2012) Spectral discrimination of cerebral amyloid lesions after peripheral application of luminescent conjugated oligothiophenes. Am J Pathol 181:1953-60 (Abstract)
Hefendehl JK, Milford D, Eicke D, Wegenast-Braun BM, Calhoun ME, Grathwohl SA, Jucker M, Liebig C (2012) Repeatable target localization for long-term in vivo imaging of mice with 2-photon microscopy. J Neurosci Meth 205:357-63 (Abstract)
Hefendehl JK*, Wegenast-Braun BM*, Liebig C, Eicke D, Milford D, Calhoun ME, Kohsaka S, Eichner M, Jucker M (2011) Long-term in vivo imaging of β-amyloid plaque appearance and growth in a mouse model of cerebral β-amyloidosis. J Neuroscience 31:624-9 (Abstract)