10 Sep 2005

This workshop has an enduring interest in facilitating the import into AD research of new technologies developed in other areas. This year an approach was presented that makes it possible to watch neurons grow and change over time inside living mice. Research on diseases that have a long prodromal phase of asymptomatic pathology and then slow progression could benefit especially from in situ biology. Frequently in AD research, interpretation of a given result is hobbled by circular arguments about whether the result is a cause of pathogenesis or a consequence of it. This problem can keep old questions unsettled despite large amounts of research. In such instances, it could be tremendously useful to have tools to observe a disease process over time in a living, behaving animal.

The green fluorescent protein of the jellyfish species Aequorea has become widely used in various genetic constructs throughout cell biology. Few people realize, however, how much more powerful this technology has become with the production of a wealth of different protein variants that fluoresce with different wavelengths. Jeff Lichtman of Harvard University gave the audience a taste of what these proteins can do. In collaboration with Joshua Sanes, Lichtman's laboratory have made more than 90 lines of mice that express GFP and similar fluorescent proteins (XFPs) in different subsets of neural cells. Most use a regulatory element from the Thy-1 promoter that drives neural expression, and indeed these mice are already in use in AD studies (Brendza et al., 2003).

In the peripheral nervous system, it is possible to counterstain the neuromuscular synapses of Thy-1/XFP mice, and then watch a nerve as it grows to innervate its muscle. One can crush the nerve and observe how single axons abandon their synapses and withdraw through their myelin tube (Walsh and Lichtman, 2003; Bishop et al., 2004), and then watch regenerating axons cross the injury site and grow back through the inside of their original Schwann cell tube to re-innervate their old target (Nguyen et al., 2002). Moreover, scientists can image changes in the branching pattern and other characteristics of a given nerve as the animal ages normally or with disease (Schaefer et al., 2005).

In a more recent strain called "Brainbow" mice, different neurons within a given nerve will light up in a multitude of rainbow colors depending on how each cell recombines a complex transgenic construct encoding four different-colored fluorescent proteins (manuscript in preparation). It is up to clever Alzheimer researchers now to devise meaningful experiments for this brilliant resource. This summary closes with a nudge and cheers to card-carrying technology geeks who'd like to develop ways to image fluorescent neuron dynamics in the CNS of live, aging mammals, preferably through the intact skull.

—Gabrielle Strobel

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References

Paper Citations

1. Brendza RP, O'Brien C, Simmons K, McKeel DW, Bales KR, Paul SM, Olney JW, Sanes JR, Holtzman DM. PDAPP; YFP double transgenic mice: a tool to study amyloid-beta associated changes in axonal, dendritic, and synaptic structures. J Comp Neurol. 2003 Feb 17;456(4):375-83. PubMed.
2. Walsh MK, Lichtman JW. In vivo time-lapse imaging of synaptic takeover associated with naturally occurring synapse elimination. Neuron. 2003 Jan 9;37(1):67-73. PubMed.
3. Bishop DL, Misgeld T, Walsh MK, Gan WB, Lichtman JW. Axon branch removal at developing synapses by axosome shedding. Neuron. 2004 Nov 18;44(4):651-61. PubMed.
4. Nguyen QT, Sanes JR, Lichtman JW. Pre-existing pathways promote precise projection patterns. Nat Neurosci. 2002 Sep;5(9):861-7. PubMed.
5. Schaefer AM, Sanes JR, Lichtman JW. A compensatory subpopulation of motor neurons in a mouse model of amyotrophic lateral sclerosis. J Comp Neurol. 2005 Sep 26;490(3):209-19. PubMed.

Further Reading

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