The triplet repeat disease X-linked spinal and bulbar muscular atrophy (SBMA) is caused by a polyglutamine expansion in the androgen receptor (AR) gene. Just how this expansion causes degeneration of lower motor neurons is unclear, but in today's Neuron, Albert La Spada and colleagues at the University of Washington Medical Center, Seattle, point an accusing finger at dysregulation of the gene for vascular epithelial growth factor (VEGF).
Previous animal models of SBMA have revealed that the polyglutamine-expanded AR must move to the nucleus in order to cause toxicity (see for example Katsuno et al., 2002), but none of these models used constructs in which the receptor is regulated by its normal promoter elements. To create such a model, first author Bryce Sopher and colleagues created mice in which human androgen receptor with short (20 repeats, AR20) and long (100 repeats, AR100) expansions are expressed from yeast artificial chromosomes.
When Sopher examined these animals, he found that their motor neurons progressively lost function; this caused gait problems, muscle weakness, and eventually paralyzed the mice. This was more pronounced in animals with the longer repeat, and virtually absent in females, in keeping with the X-linked pattern of inheritance and sex-limited expression of the human disease. Overall, the model recapitulates SBMA more faithfully than previous ones did, the authors claim.
Sopher found that the expanded receptor does indeed end up in the nucleus, but curiously, it does not form aggregates detectable by microscope in motor neurons, though small soluble aggregates might have been there. Instead, the authors detected aggregates in spinal cord astrocytes in the AR100 transgenics. (It is worth noting that recent evidence has shown that mutant superoxide dismutase, which causes familial amyotrophic lateral sclerosis (ALS), need not be expressed in neurons to cause disease. See ARF related news story).
To test the role of the expanded receptor in regulating transcription, Sopher and colleagues built on previous observations that the AR might interfere with CREB binding protein (CBP), a well-studied transcriptional coactivator (see McCampbell et al., 2000). When the authors immunoprecipitated CBP, they found that the androgen receptor came along for the ride, and the longer the polyglutamine repeat, the stronger the association between the two proteins.
So what might be the consequences of this liaison? Could expanded AR prevent CBP from activating gene transcription? And which affected gene(s) may be important for motor neuron survival? To answer these questions, Sopher turned to a CBP's downstream target, VEGF. This growth factor is known to be regulated by a CBP binding element. Previous work by Peter Carmeliet’s group in Belgium has shown that abolishing that element induces motor neuron degeneration (Oosthuyse et al., 2001; see also Lambrechts et al., 2003). Contrary to expectations, when Sopher measured VEGF expression in the AR20 and AR100 neurons, it was no different from controls. However, a particular isoform of VEGF—VEGF 164—has been shown to have neurotrophic activity, and when Sopher measured expression of this isoform, he indeed saw a progressive loss of expression. AR100 transgenics had about 30 and 45 percent less VEGF than wild-type mice at 6.5 and 11 months of age, respectively. Moreover, when the authors added VEGF 164 to cultured neurons expressing expanded androgen receptor, the growth factor rescued them: 60 percent of the cells died in the absence of VEGF, versus 20 percent in its presence.
The authors suggest that "decreased expression of VEGF or an inability to upregulate VEGF in the face of injury, ischemia, or stress is a fundamental property of degenerating motor neurons." Note, too, that VEGF has been linked with increased risk for developing amyotrophic lateral sclerosis.—Tom Fagan
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No Available Further Reading
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