05 Dec 2003

Two new approaches for modeling neurodegeneration in mice were unveiled at the 33rd Annual meeting of the Society for Neuroscience in New Orleans. Frank LaFerla’s group at University of California, Irvine, generated mice in which selective subpopulations of neurons can be ablated in a time-dependent manner (Abstract 773.13; view abstracts at the SfN/ScholarOne website). Most methods to lesion-specific neuronal subfields rely on excitotoxicity; they act on relatively nonspecific populations of neurons and are hard to control temporally. LaFerla’s group combined inducible transgenic expression of the diphtheria toxin A chain (DT-A)—a potent cytotoxin in eukaryotic cells—with the binary tetracycline (tet-off) promoter system to selectively eliminate Ca2+-calmodulin kinase II (CaMKII)-expressing neurons in transgenic mice in a time-dependent manner. Induction of DT-A transgene expression resulted in a selective and focal loss of neurons in specific brain regions, with CA1 hippocampal neurons emerging as the most vulnerable population, followed by the neocortex, dentate gyrus, striatum, and basal forebrain. The CA3 region remained intact and was affected only after extreme periods of DT-A expression. The DT-A expression was clean, with no leaking during development when the transgene is repressed; in this way, the transgenic mice develop normally and lesions on adult brains can be assessed directly. This system can now be used to explore a number of questions on the consequences of focal cell loss. Since DT-A expression can be turned off at will, one can look at recovery after the lesion and use various manipulations or putative therapeutics to increase recovery. Jason Shepherd is a coauthor of this study.

Wouldn’t it be nice to have a live biosensor, a way to image neurodegeneration and cellular injury without having to kill the mice? Tony Wyss-Coray’s group at Stanford University developed a new transgenic model to do just that (Abstract 773.14). The cytokine TGF-β1 is known to be increased in AD and is also rapidly induced in response to injury, for example, as triggered by the toxin kainate. TGF-β1 signals via Smad, which translocates to the nucleus, binds to Smad binding elements (SBEs), and in this way activates transcription of various genes. Taking advantage of this, Wyss-Coray’s group engineered mice that overexpress an SBE-luciferase reporter gene. The luciferase protein bioluminesces in the presence of its substrate luciferin (which can be injected), and an ultrasensitive, cooled CCD camera can capture this bioluminescence inside the brain of mice. TGF-1 activity, therefore, turned on luciferase, making the mice “glow.” The scientists found that TGF-β1 activity was highest in the brain, specifically in the hippocampus and cortex. After intraperitoneal injection of kainate, luciferase activity showed a biphasic induction at one hour and 24 hours after injection. The researchers also showed that the reporter gene could be imaged in live, anaesthetized animals in real time. These mice can be used to assess the progression and outcome of neurodegenerative disorders. The consequences of chronic degeneration in these animals need to be further assessed, Wyss-Coray added. These mice could be crossed to mouse models of neurodegeneration, and could help in assessing the role of inflammation in various disease paradigms.—Jason Shepherd is a graduate student at Johns Hopkins University School of Medicine, Baltimore, Maryland.